This value can not be relied on as it doesn't work for
multi-configuration generators. I don't think this undocumented option
is used much, if at all, so I think we should remove it.

Problem: On tmux v3.2+, the `terminal-features` option may be used to enable RGB
capabilities over `terminal-overrides`. However, `show-messages` cannot be used
to detect if RGB capabilities are enabled using `terminal-features`.

Solution: Try to use `display-message -p #{client_termfeatures}` instead.
The returned features include "RGB" if either "RGB" is set in
`terminal-features`, or if "Tc" or "RGB" is set in `terminal-overrides` (as
before).
Nothing is returned by tmux versions older than v3.2, so fallback to checking
`show-messages` in that case.

Also, un-Vimscriptify the previous logic a bit, and change the error message to
point to using the `terminal-features` option instead for newer tmux versions.

Regression from the health.vim to .lua changes.

Unlike Vim script, Lua does not implicitly convert strings to numbers, so this
comparison threw an error.

* refactor: remove all vimscript from nvim/health
* fixup: previous method broke if you had a folder named 'x-lua'

Co-authored-by: Gustavo Sampaio <gbritosampaio@gmail.com>
Co-authored-by: C.D. MacEachern <craig.daniel.maceachern@gmail.com>
Co-authored-by: Sean Dewar <seandewar@users.noreply.github.com>
Co-authored-by: Tomas Nemec <nemi@skaut.cz>

- If Nvim was just started, don't create a new tab.
- Name the buffer "health://".
- Use "help" syntax instead of "markdown". It fits better, and
  eliminates various workarounds.
- Simplfy formatting, avoid visual noise.
- Don't print a "INFO" status, it is noisy.
- Drop the ":" after statuses, they are already UPPERCASE and highlighted.

Move man/health.lua into the "runtime" check.

fix #20696