feat(defaults): pretty :help headings #30544

Problem:
Headings in :help do not stand out visually.

Solution:
Define a non-standard `@markup.heading.1.delimiter` group and
special-case it in `highlight_group.c`.

FUTURE:
This is a cheap workaround until we have #25718 which will enable:
- fully driven by `vimdoc/highlights.scm` instead of using highlight
  tricks (`guibg=bg guifg=bg guisp=fg`)
- better support of "cterm" ('notermguicolors')

runtime/syntax/help.vim

@@ -15,7 +15,7 @@ set cpo&vim
 syn match helpHeadline		"^[A-Z.][-A-Z0-9 .,()_']*?\=\ze\(\s\+\*\|$\)"
 syn match helpSectionDelim	"^===.*===$"
 syn match helpSectionDelim	"^---.*--$"
-" Neovim: support language annotation in codeblocks
+" Nvim: support language annotation in codeblocks
 if has("conceal")
   syn region helpExample	matchgroup=helpIgnore start=" >[a-z0-9]*$" start="^>[a-z0-9]*$" end="^[^ \t]"me=e-1 end="^<" concealends
 else