Uses an overload to properly show the different return type based on the
input parameter.

Problem:
`yx` uses "y" prefix, which shadows a builtin operator.

Solution:
Use `g=` (in the form of `g==` currently), drawing from precedent of
CTRL-= and 'tpope/vim-scriptease'.

`yxx` in Normal mode over a Lua or Vimscript code block section will execute the code.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

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')

- Don't assume b:undo_ftplugin is set when first modifying it.
- Don't assume b:undo_ftplugin already contains some resetting.

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 '|'.

Related to #29506, but adding vim.treesitter.stop() to b:undo_ftplugin
doesn't solve the problem yet.

Problem: Creating the table of contents for `gO` is complicated.

Solution: Use treesitter instead.

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).