Provide library for other actions to use

[hazelweakly]

This is the already existing PR from actions/setup-haskell.

@omelkonian, I quite liked your work with this. Unfortunately, a few things in the structure of the repo have changed enough to make porting it over a non-automatic process. It shouldn't be too bad, and I'll help out with it, but I just wanted to give you a heads up with that.