Add some status checks through GitHub Actions

It would be interesting to add some status checks through GitHub Actions for
various things, such as testing whether Zsh would accept the configuration lying
herein, whether the latest (Git) version of XMonad, coupled with
`xmonad-contrib`, would compile the configuration contained in this repository,
etc.

Linting, syntax checking, testing (if there can even be such a thing) would be
interesting to put in here, despite the configuration beind actively tested on
the target environment before ever being committed.