Problem:
`:checkhealth nvim` warns about missing vimrc if `init.lua` exists but
`init.vim` does not.

Solution:
Check for any of: init.vim, init.lua, $MYVIMRC.
Fix #25291

The following functions are deprecated and will be removed in
Nvim v0.11:

- health#report_start()
- health#report_info()
- health#report_ok()
- health#report_warn()
- health#report_error()
- vim.health.report_start()
- vim.health.report_info()
- vim.health.report_ok()
- vim.health.report_warn()
- vim.health.report_error()

Users should instead use these:

- vim.health.start()
- vim.health.info()
- vim.health.ok()
- vim.health.warn()
- vim.health.error()

* feat(lua): vim.tbl_contains supports general tables and predicates

Problem: `vim.tbl_contains` only works for list-like tables (integer
keys without gaps) and primitive values (in particular, not for nested
tables).

Solution: Rename `vim.tbl_contains` to `vim.list_contains` and add new
`vim.tbl_contains` that works for general tables and optionally allows
`value` to be a predicate function that is checked for every key.

In a few places ipairs was used to iterate over elements of the array.
However, the first return value of ipairs was erronously used, which is
not the value, but rather the index. This would result in errors, for
instance when trying to retrieve a field from the value.

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>