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

CHANGELOG.md

@@ -73,6 +73,8 @@
   code)
 - Highlights will also take into account `ctermfg/bg` when setting default values.
   This ensures outline.nvim highlights work if `termguicolors` is not enabled
+- A built-in provider for `norg` files that displays headings in the outline is now
+  provided. This requires `norg` parser to be installed for treesitter
 
 ### Fixes