feat: Norg provider and support of external providers

- Closes #3
- Ref: simrat39/symbols-outline.nvim#190

Norg contains indents and different types of verbatim tags, I was rather
lazy to read the spec properly and parse norg using regex line-by-line
like markdown, so used treesitter instead. The only requirement is the
`norg` parser for treesitter to be installed. Tested on nvim 0.7.2.

This should lead the way for supporting vimdoc files in a similar
manner.

Documentation for how external providers could look like as of now has
been added.

In the future we could let the provider determine what to do for each
keymap, such as `goto_location` and `toggle_preview`. This would allow
the zk extension[1] to work properly without having to override existing
functions (bad practice).

[1]: https://github.com/mickael-menu/zk-nvim/discussions/134

lua/outline/providers/norg.lua

@@ -0,0 +1,135 @@
+local M = {
+  name = 'norg',
+  query = [[
+  [
+  (heading1 (heading1_prefix)
+    title: (paragraph_segment) @name)
+  (heading2 (heading2_prefix)
+    title: (paragraph_segment) @name)
+  (heading3 (heading3_prefix)
+    title: (paragraph_segment) @name)
+  (heading4 (heading4_prefix)
+    title: (paragraph_segment) @name)
+  (heading5 (heading5_prefix)
+    title: (paragraph_segment) @name)
+  (heading6 (heading6_prefix)
+    title: (paragraph_segment) @name)
+  ]
+  ]],
+}
+
+function M.supports_buffer(bufnr)
+  if vim.api.nvim_buf_get_option(bufnr, 'ft') ~= 'norg' then
+    return false
+  end
+
+  local status, parser = pcall(vim.treesitter.get_parser, bufnr, 'norg')
+  if not status or not parser then
+    return false
+  end
+
+  M.parser = parser
+  return true
+end
+
+local is_ancestor = vim.treesitter.is_ancestor
+
+if not _G._outline_nvim_has[8] then
+  is_ancestor = function(dest, source)
+    if not (dest and source) then
+      return false
+    end
+
+    local current = source
+    while current ~= nil do
+      if current == dest then
+        return true
+      end
+
+      current = current:parent()
+    end
+
+    return false
+  end
+end
+
+local function rec_remove_field(node, field)
+  node[field] = nil
+  if node.children then
+    for _, child in ipairs(node.children) do
+      rec_remove_field(child, field)
+    end
+  end
+end
+
+function M.request_symbols(callback, opts)
+  if not M.parser then
+    local status, parser = pcall(vim.treesitter.get_parser, 0, 'norg')
+
+    if not status or not parser then
+      callback(nil, opts)
+      return
+    end
+
+    M.parser = parser
+  end
+
+  local root = M.parser:parse()[1]:root()
+  if not root then
+    callback(nil, opts)
+    return
+  end
+
+  local r = { children = {}, tsnode = root, name = 'root' }
+  local stack = { r }
+
+  local query
+  if _G._outline_nvim_has[9] then
+    query = vim.treesitter.query.parse('norg', M.query)
+  else
+    ---@diagnostic disable-next-line: deprecated
+    query = vim.treesitter.query.parse_query('norg', M.query)
+  end
+  ---@diagnostic disable-next-line: missing-parameter
+  for _, captured_node, _ in query:iter_captures(root, 0) do
+    local row1, col1, row2, col2 = captured_node:range()
+    local title = vim.api.nvim_buf_get_text(0, row1, col1, row2, col2, {})[1]
+    local heading_node = captured_node:parent()
+    row1, col1, row2, col2 = heading_node:range()
+
+    title = title:gsub('^%s+', '')
+
+    local current = {
+      kind = 15,
+      name = title,
+      selectionRange = {
+        start = { character = col1, line = row1 },
+        ['end'] = { character = col2, line = row2 - 1 },
+      },
+      range = {
+        start = { character = col1, line = row1 },
+        ['end'] = { character = col2, line = row2 - 1 },
+      },
+      children = {},
+      tsnode = heading_node,
+    }
+
+    while #stack > 0 do
+      local top = stack[#stack]
+      if is_ancestor(top.tsnode, heading_node) then
+        current.parent = top
+        table.insert(top.children, current)
+        break
+      end
+      table.remove(stack, #stack)
+    end
+
+    table.insert(stack, current)
+  end
+
+  rec_remove_field(r, 'tsnode')
+
+  callback(r.children, opts)
+end
+
+return M