Migrate to GitHub Actions for CI

[thomashoneyman]

Migrates to GitHub Actions for CI. This serves as a demonstration for other repositories in the PureScript core organization which have tests (unlike the same change made for semirings, as that library doesn't have tests).

[thomashoneyman]

If desired, I can also add caching for NPM / Bower dependencies, as well as caching for the output directory for the built PureScript code. It's not a huge deal -- the entire build / test pipeline takes about 30 seconds -- but it's easy to add.

[hdgarrood]

Thanks! I’d suggest not bothering with caching since it’s another thing to potentially go wrong and it’s not saving us a whole lot of time in this case.

[thomashoneyman]

Backlinking to purescript/purescript#3962