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/queries/vimdoc/highlights.scm

@@ -1,9 +1,9 @@
 (h1
-  (delimiter) @markup.heading.1
+  (delimiter) @markup.heading.1.delimiter
   (heading) @markup.heading.1)
 
 (h2
-  (delimiter) @markup.heading.2
+  (delimiter) @markup.heading.2.delimiter
   (heading) @markup.heading.2)
 
 (h3