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')
It seems that nvim-treesitter stops treesitter highlight when changing
filetype, so it makes sense for builtin ftplugins to do this as well.
Use :call and v:lua here to allow separation with '|'.
Problem: Not all default highlight groups show their actual colors.
Solution: Refactor `vimhelp.lua` and apply it to all relevant lists (UI
groups, syntax groups, treesitter groups, LSP groups, diagnostic groups).
- Add runtime/lua/vim/vimhelp.lua, which is a translation of Vim's
runtime/import/dist/vimhelp.vim.
- Unlike Vim, run the highlighting from an ftplugin file instead of a
syntax file, so that it is run even if using treesitter.
