MAJOR: Complete rewrite of outline parsing into buffer lines

Scope:
- Parsing of symbol tree
- Producing the flattened tree
- Producing the lines shown in the outline based on the symbol tree
- API of exported functions for parser.lua and writer.lua

Note that the formatting of the outline remains the same as before.

Fixes:
- Guide highlights sometimes cover fold marker areas (may be related to
  the issue brought up by @silvercircle on reddit)
- Guide highlights do not work when using guide markers of different
  widths than the default (such as setting all markers to ascii chars)

All of these issues are now fixed after integrating the a parser
algorithm.

This commit introduces:
1. A better algorithm for flattening & parsing the tree in one go
2. `OutlineFoldMarker` highlight group
3. Fixed inconsistent highlighting of guides and legacy (somewhat weaker
   code), through (1).
4. Minor performance improvements
5. Type hints for the symbol tree
6. Removed several functions from writer.lua and parser.lua due to them
   being merged into writer.make_outline

This can be seen as a breaking change because functions that were
exported had altered behaviours. However I doubt these functions
actually have any critical use outside of this plugin, hence it isn't
really a breaking change as the user-experience remains the same.

The extraneous left padding on the outline window is now a relic of the
past 🎉

The old implementation, parser.get_lines used a flattened tree and was
inefficient, full of off-by-one corrections.

While trying to look for bug fixes in that function I realized it's the
sort of "if it works, don't touch it" portion of code.

Hence, I figured a complete rewrite is necessary.

Now, the function responsible for making the outline lines lives at
writer.make_outline. Building the flattened tree, getting lines, details
and linenos are now done in one go.

This is a tradeoff between modular design and efficiency.

parser.lua still serve important purposes:
- local parse_result function converts the hierarchical tables from
  provider into a nested form tree, used everywhere in outline.nvim. The
  type hint of the return value is now defined -- outline.SymbolNode
- preorder_iter is an iterator that traverses the aforementioned tree in
  pre-order style. First the parents, all the childrens, and so on until
  the last node of the root tree. This is used in writer.make_outline to
  abstract a way the traversal code from the code of making the lines.

Thanks to stack overflow I did not have to consult a DS book to figure
out the cleanest way of this traversal method without using recursion.

This, of course, closes #14 on github.

Note that another minor 'breaking' change is that previously, hl for the
guides where grouped per-character, now they are grouped together for
all the guide markers in the same line. This should not be a problem for
those who only style the fg color for guide hl. However, if you're
styling the bg color, they will now take on that bg collectively rather
than individually.

This change eliminates future maintenance burden because controlling
per-character guide highlights requires careful avoidance of off-by-one
errors.

I have tested most common features to work as before.
I may have missed particular edge cases.

Please take note of "scope" at the top of this commit message.

lua/outline/init.lua

@@ -43,7 +43,9 @@ end
 -- STATE
 -------------------------
 M.state = {
+  ---@type outline.SymbolNode[]
   outline_items = {},
+  ---@type outline.FlatSymbolNode[]
   flattened_outline_items = {},
   code_win = 0,
   -- In case unhide_cursor was called before hide_cursor for _some_ reason,
@@ -56,10 +58,10 @@ local function wipe_state()
 end
 
 local function _update_lines()
-  M.state.flattened_outline_items = parser.flatten(M.state.outline_items)
-  writer.parse_and_write(M.view.bufnr, M.state.flattened_outline_items)
+  M.state.flattened_outline_items = writer.make_outline(M.view.bufnr, M.state.outline_items)
 end
 
+---@param items outline.SymbolNode[]
 local function _merge_items(items)
   utils.merge_items_rec(
     { children = items },
@@ -90,11 +92,13 @@ end
 
 M._refresh = utils.debounce(__refresh, 100)
 
+---@return outline.FlatSymbolNode
 function M._current_node()
   local current_line = vim.api.nvim_win_get_cursor(M.view.winnr)[1]
   return M.state.flattened_outline_items[current_line]
 end
 
+---@param change_focus boolean
 function M.__goto_location(change_focus)
   local node = M._current_node()
   vim.api.nvim_win_set_cursor(
@@ -107,7 +111,9 @@ function M.__goto_location(change_focus)
   end
 end
 
--- Wraps __goto_location and handles auto_close
+---Wraps __goto_location and handles auto_close.
+---@see __goto_location
+---@param change_focus boolean
 function M._goto_location(change_focus)
   M.__goto_location(change_focus)
   if change_focus and cfg.o.outline_window.auto_close then
@@ -120,6 +126,7 @@ function M._goto_and_close()
   M.close_outline()
 end
 
+---@param direction "up"|"down"
 function M._move_and_goto(direction)
   local move = direction == 'down' and 1 or -1
   local cur = vim.api.nvim_win_get_cursor(0)
@@ -128,6 +135,8 @@ function M._move_and_goto(direction)
   M.__goto_location(false)
 end
 
+---@param move_cursor boolean
+---@param node_index integer Index for M.state.flattened_outline_items
 function M._toggle_fold(move_cursor, node_index)
   local node = M.state.flattened_outline_items[node_index] or M._current_node()
   local is_folded = folding.is_folded(node)
@@ -186,6 +195,9 @@ local function setup_buffer_autocmd()
   end
 end
 
+---@param folded boolean
+---@param move_cursor? boolean
+---@param node_index? integer
 function M._set_folded(folded, move_cursor, node_index)
   local node = M.state.flattened_outline_items[node_index] or M._current_node()
   local changed = (folded ~= folding.is_folded(node))
@@ -212,6 +224,7 @@ function M._set_folded(folded, move_cursor, node_index)
   end
 end
 
+---@param nodes outline.SymbolNode[]
 function M._toggle_all_fold(nodes)
   nodes = nodes or M.state.outline_items
   local folded = true
@@ -226,6 +239,8 @@ function M._toggle_all_fold(nodes)
   M._set_all_folded(not folded, nodes)
 end
 
+---@param folded boolean|nil
+---@param nodes? outline.SymbolNode[]
 function M._set_all_folded(folded, nodes)
   local stack = { nodes or M.state.outline_items }
 
@@ -242,6 +257,7 @@ function M._set_all_folded(folded, nodes)
   _update_lines()
 end
 
+---@param winnr? integer Window number of code window
 function M._highlight_current_item(winnr)
   local has_provider = M.has_provider()
   local has_outline_open = M.view:is_open()
@@ -373,6 +389,8 @@ local function setup_keymaps(bufnr)
   end)
 end
 
+---@param response table?
+---@param opts outline.OutlineOpts?
 local function handler(response, opts)
   if response == nil or type(response) ~= 'table' or M.view:is_open() then
     return
@@ -394,9 +412,7 @@ local function handler(response, opts)
   local items = parser.parse(response)
 
   M.state.outline_items = items
-  M.state.flattened_outline_items = parser.flatten(items)
-
-  writer.parse_and_write(M.view.bufnr, M.state.flattened_outline_items)
+  M.state.flattened_outline_items = writer.make_outline(M.view.bufnr, items)
 
   M._highlight_current_item(M.state.code_win)
 
@@ -405,9 +421,12 @@ local function handler(response, opts)
   end
 end
 
+---@class outline.OutlineOpts
+---@field focus_outline boolean
+
 ---Set position of outline window to match cursor position in code, return
 ---whether the window is just newly opened (previously not open).
----@param opts table? Field `focus_outline` = `false` or `nil` means don't focus on outline window after following cursor. If opts is not provided, focus will be on outline window after following cursor.
+---@param opts outline.OutlineOpts? Field `focus_outline` = `false` or `nil` means don't focus on outline window after following cursor. If opts is not provided, focus will be on outline window after following cursor.
 ---@return boolean ok Whether it was successful. If ok=false, either the outline window is not open or the code window cannot be found.
 function M.follow_cursor(opts)
   if not M.view:is_open() then
@@ -449,7 +468,8 @@ end
 
 ---Toggle the outline window, and return whether the outline window is open
 ---after this operation.
----@param opts table? Table of options, @see open_outline
+---@see open_outline
+---@param opts outline.OutlineOpts? Table of options
 ---@return boolean is_open Whether outline window is open
 function M.toggle_outline(opts)
   if M.view:is_open() then
@@ -471,7 +491,7 @@ local function _cmd_toggle_outline(opts)
 end
 
 ---Open the outline window.
----@param opts table? Field focus_outline=false means don't focus on outline window after opening. If opts is not provided, focus will be on outline window after opening.
+---@param opts outline.OutlineOpts? Field focus_outline=false means don't focus on outline window after opening. If opts is not provided, focus will be on outline window after opening.
 function M.open_outline(opts)
   if not opts then
     opts = { focus_outline = true }