Add `:checkhealth freeze` for required and optional depencies

[AlejandroSuero]

Motivation

I am doing my own plugin for freeze, called freeze-code.nvim and one of the things that almost all plugins that requires dependencies to run, in my plugin I have more because I handle also installation in case that freeze is not installed.

Example

:checkhealth freeze-code:

In my plugin, it will check freeze as required and copy, open and install commands as optional, depending on the OS. For MacOS xclip is not required but it could be used, so It shows as WARNING not installed and tells the user that by installing it will have extended capabilities.

In my case if I was on Windows it will show Expand-Archive instead of tar to extract archive for example.