- Complete function:
  There was lots of unnecessary C code for the complete function, therefore
  moving it to Lua and use all the plumbing we have in place to retrieve the
  results.
- Moving the module:
  It's important we keep nvim lua modules name spaced, avoids conflict with
  plugins, luarocks, etc.

* fix(runtime/health): mitigate issues with duplicate healthchecks
  Previously if a healthcheck was found as Lua and Vim it was executed
  both times.
  This new implementations prefers Lua, therefore if two are found It only
  runs the Lua one, this way a plugin can mantain both implementations the
  Lua one with the method `check()` and the autoload function `#check()`
  (for none HEAD nvim versions).
  **Note: This will require plugins to use `check()` as the function name,
  since the autoload function that wraps the lua implementation won't be
  called**
* docs(health): use spaces and don't overuse backtics

followup to #15259

- Add tests for lua healthchecks (failure, success and submodules).
- Reword some of the test naming for improved logs readability.
- Modify render test to accomodate the changes of the health autoload function.
- Add test for :checkhealth completion of Lua healtchecks.