feat(runtime): highlight hl groups in syntax.txt (#25050)

- 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.

runtime/ftplugin/help.lua

@@ -0,0 +1,3 @@
+if vim.endswith(vim.fs.normalize(vim.api.nvim_buf_get_name(0)), '/doc/syntax.txt') then
+  require('vim.vimhelp').highlight_groups()
+end