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.
