switch to github actions; test on a matrix; run eslint in CI

[ljharb]

This PR:

• switches from travis-ci (which has become unreliable anyways) to Github Actions
• tests in all supported node majors
• adds eslint and runs it in CI as well (but only once; the tests run on each node version)

Note that the actions may not run on this PR until they're in the master branch of this repo, but you can view the results here: https://github.com/ljharb/language-tags/actions (you'll note the tests fail solely because coveralls isn't enabled on my fork)

This is the first PR to address #15 (comment); after this lands, I'll put up another that adds babel and restores older engines compatibility.

After this merges, you can add linting and tests as required status checks.

[mattcg]

Thank you for this!

[ljharb]

@mattcg whoops, turns out i should be using the coveralls github action instead of | coveralls; the next PR will fix that.

[ljharb]

#18 fixes coverage; still working on the babel change.

[ljharb]

→ #19