Move CI from Travis CI to GitHub Actions

[nazar-pc]

Travis CI is disabled for this repo and it would be easier for everyone involved to just migrate it over to GitHub Actions.

The only non-trivial thing might be with code coverage.

[mvines]

💯 Travis CI needs to be avoided like the plague. There's major problems in that organization these days. Their response to this recent incident in particular was unacceptable: https://cycode.com/blog/github-oauth-compromise-affecting-heroku-and-travis-ci-users/