feat(treesitter): table of contents for checkhealth, markdown (#32282)

Problem: It's difficult to navigate large structured text files (vim
help, checkhealth, Markdown).

Solution: Support `gO` for table of contents and `]]`/`[[` for moving
between headings for all these filetypes using treesitter queries.

Refactor: colorization of highlight groups is moved to the `help` ftplugin
while headings-related functionality is implemented in a private
`vim.treesitter` module for possible future use for other filetypes.

runtime/lua/vim/treesitter/language.lua

@@ -5,6 +5,7 @@ local M = {}
 ---@type table<string,string>
 local ft_to_lang = {
   help = 'vimdoc',
+  checkhealth = 'vimdoc',
 }
 
 --- Returns the filetypes for which a parser named {lang} is used.