feat(help): use treesitter for table of contents

Problem: Creating the table of contents for `gO` is complicated.

Solution: Use treesitter instead.

runtime/ftplugin/help.vim

@@ -21,77 +21,5 @@ endif
 " Prefer Vim help instead of manpages.
 setlocal keywordprg=:help
 
-if !exists('g:no_plugin_maps')
-  function! s:show_toc() abort
-    let bufname = bufname('%')
-    let info = getloclist(0, {'winid': 1})
-    if !empty(info) && getwinvar(info.winid, 'qf_toc') ==# bufname
-      lopen
-      return
-    endif
-
-    let toc = []
-    let lnum = 2
-    let last_line = line('$') - 1
-    let last_added = 0
-    let has_section = 0
-    let has_sub_section = 0
-
-    while lnum && lnum <= last_line
-      let level = 0
-      let add_text = ''
-      let text = getline(lnum)
-
-      if text =~# '^=\+$' && lnum + 1 < last_line
-        " A de-facto section heading.  Other headings are inferred.
-        let has_section = 1
-        let has_sub_section = 0
-        let lnum = nextnonblank(lnum + 1)
-        let text = getline(lnum)
-        let add_text = text
-        while add_text =~# '\*[^*]\+\*\s*$'
-          let add_text = matchstr(add_text, '.*\ze\*[^*]\+\*\s*$')
-        endwhile
-      elseif text =~# '^[A-Z0-9][-A-ZA-Z0-9 .][-A-Z0-9 .():]*\%([ \t]\+\*.\+\*\)\?$'
-        " Any line that's yelling is important.
-        let has_sub_section = 1
-        let level = has_section
-        let add_text = matchstr(text, '.\{-}\ze\s*\%([ \t]\+\*.\+\*\)\?$')
-      elseif text =~# '\~$'
-            \ && matchstr(text, '^\s*\zs.\{-}\ze\s*\~$') !~# '\t\|\s\{2,}'
-            \ && getline(lnum - 1) =~# '^\s*<\?$\|^\s*\*.*\*$'
-            \ && getline(lnum + 1) =~# '^\s*>\?$\|^\s*\*.*\*$'
-        " These lines could be headers or code examples.  We only want the
-        " ones that have subsequent lines at the same indent or more.
-        let l = nextnonblank(lnum + 1)
-        if getline(l) =~# '\*[^*]\+\*$'
-          " Ignore tag lines
-          let l = nextnonblank(l + 1)
-        endif
-
-        if indent(lnum) <= indent(l)
-          let level = has_section + has_sub_section
-          let add_text = matchstr(text, '\S.\{-}\ze\s\=\~$')
-        endif
-      endif
-
-      let add_text = substitute(add_text, '\s\+$', '', 'g')
-      if !empty(add_text) && last_added != lnum
-        let last_added = lnum
-        call add(toc, {'bufnr': bufnr('%'), 'lnum': lnum,
-              \ 'text': repeat("\u00a0\u00a0", level) . add_text})
-      endif
-      let lnum = nextnonblank(lnum + 1)
-    endwhile
-
-    call setloclist(0, toc, ' ')
-    call setloclist(0, [], 'a', {'title': 'Help TOC'})
-    lopen
-    let w:qf_toc = bufname
-  endfunction
-
-  nnoremap <silent><buffer> gO :call <sid>show_toc()<cr>
-endif
-
 let &cpo = s:cpo_save
 unlet s:cpo_save