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')
09d76afe84
