feat(runtime/health): support lua healthchecks

- Refactor health.vim to discover lua healthcheck in the runtime
  directories lua/**/health{/init}.lua
- Support healthchecks for lua submodules e.g :checkhealth vim.lsp and
  also support wildcard "*" at the end for all submodules
  :checkhealth vim*
- Refactor health.vim to use variable scope instead of output capturing
- Create health.lua module to wrap report functions and future
  extensibility.
- Move away from searching just in the runtimepath, use
  `nvim_get_runtime_file` due to #15632

Example:
Plugin linter in rtp can declare it's checkhealts in lua module
`lua/linter/health{/init}.lua` that returns a table with a method
"check" that when executed calls the report functions provided by the
builtin lua module require("health").

The plugin also has a submodule `/lua/linter/providers` in which it
defines `/lua/linter/providers/health{/init}.lua`

This plugin healthcheck can now be run by the ex command:
  `:checkhealth linter linter.providers`

Also calling all submodules can be done by:
  `:checkhealth linter*

And "linter" and "linter.provider" would be discovered when:
  `:checkhealth`

runtime/lua/health.lua

@@ -0,0 +1,23 @@
+local M = {}
+
+function M.report_start(msg)
+  vim.fn['health#report_start'](msg)
+end
+
+function M.report_info(msg)
+  vim.fn['health#report_info'](msg)
+end
+
+function M.report_ok(msg)
+  vim.fn['health#report_ok'](msg)
+end
+
+function M.report_warn(msg, ...)
+  vim.fn['health#report_warn'](msg, ...)
+end
+
+function M.report_error(msg, ...)
+  vim.fn['health#report_error'](msg, ...)
+end
+
+return M