Switch to GitHub Actions as Travis is no longer available

[kisaragi-hiu]

Travis CI is no longer freely available for open source projects. This PR switches to GitHub Actions instead, now that it is a thing.

The workflow is mostly copied from https://docs.github.com/en/actions/automating-builds-and-tests/building-and-testing-nodejs.

• Uses npm install instead of npm ci as the latter does not work when there isn't a lock file.
• Test on Node 8.x, 14.x, and 20.x; 8.x will be the minimum if readdirp is updated to 3.6.0, while 14.x and 20.x are sort of arbitrarily chosen.
• I copied the coverage uploading command from .travis.yml, but left it commented out. This way, if you decide to set up coverage uploading again, at least the command is right there and you would just have to set up credentials.