Set up GitHub Actions for CI

[botandrose]

Hi @1cg, I've got something working here. Some notes:

1. CI won't run in PRs (including this one) until the main branch has the CI action.
2. So to prove that this PR does what it says it does, here's the same commit SHA running on my fork: https://github.com/botandrose/idiomorph/actions/runs/11441381562/job/31829306598
3. This test run failed, but it looks to be an accurate state of the repo, because I'm seeing this exact same result when I run it locally.
4. Once this is merged I'll write a follow-up PR getting CI green again.
5. I had to pin Node to 16 because mocha-chrome doesnt work with 17+: Is this still maintained? shellscape/mocha-chrome#46 (comment) . mocha-chrome seems to be unmaintained, so I imagine we'll want to switch it out for something else, later.