Add GitHub Actions for CI

[agilgur5]

Would love to use Dagger, but not sure that it's mature enough right now. GH Actions I think have better integration than Travis CI for OSS, esp. since https://travis-ci.org was shuttered/merged (agilgur5/react-signature-canvas#64 (comment))

[agilgur5]

Should be able to more or less duplicate agilgur5/react-signature-canvas#84 here.

Will open up a separate issue for GH Actions for releases vs. general CI, as I haven't found or fully configured an optimal fit for releases and changelog automation yet -- will want to look through that ecosystem again and test some tooling out.

[agilgur5]

Added Codecov support in 417285e as well while I was at it