ci: Add hlint workflow
ci: Add hlint workflow

The "haskell/actions/hlint-run" GitHub Action produces annotations which
GitHub shows in the code diff, in addition to just failing the workflow
when there are any suggestions. Also, now it runs in parallel to the
other workflows.

Related: https://github.com/xmonad/xmonad-contrib/issues/669
Related: https://github.com/xmonad/xmonad-contrib/commit/dd26fcc3f1c5ff16399e860502be10c4ada37b8a

xmonad/xmonad