Switch from Travis-CI to GitHub Actions, please?

[SethTisue]

Lightbend has been paying the Travis-CI bill for scala/* repos, but now we're looking to make a push to move as many repos as we can to GitHub Actions, partly so we can move to a cheaper plan with fewer concurrent jobs, and also partly just to standardize and not to have to keep knowledge of two CI systems in our collective heads.

I'm going to take care of the module repos (scala-xml, scala-parser-combinators, and the others). @sjrd can I ask that someone at the Center migrate this one?

[sjrd]

Yes, absolutely, I'll look into it or assign it ASAP.