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/config.lua

@@ -89,7 +89,7 @@ M.defaults = {
     up_and_jump = '<C-k>',
   },
   providers = {
-    priority = { 'lsp', 'coc', 'markdown' },
+    priority = { 'lsp', 'coc', 'markdown', 'norg' },
     lsp = {
       blacklist_clients = {},
     },
@@ -211,8 +211,8 @@ end
 ---@return boolean include
 function M.should_include_symbol(kind, bufnr)
   local ft = vim.api.nvim_buf_get_option(bufnr, 'ft')
-  -- There can only be one kind in markdown as of now
-  if ft == 'markdown' or kind == nil then
+  -- There can only be one kind in markdown and norg as of now
+  if ft == 'markdown' or ft == 'norg' or kind == nil then
     return true
   end