Add a CI workflow that runs the test suite

[annakrystalli]

This repo has no continuous integration of any kind. There is no .github/ directory, and no Travis, CircleCI, GitLab, Appveyor or Azure config either, so npm test has never run anywhere except on a contributor's machine. Pull requests report no checks.

The cost of that is concrete: the closestYear() failure in #71 is on master today, and it only reproduces in some timezones. A contributor in the US sees a green suite, one in Europe or Asia sees a red one, and nothing in the repo arbitrates.

Proposal

A GitHub Actions workflow running npm ci && npm test on push and pull request.

Two things worth building in from the start:

• A timezone matrix. Running the suite under at least one timezone ahead of UTC and one behind would have caught closestYear() returns a date one day early in timezones ahead of UTC #71 on the commit that introduced it. TZ is a plain environment variable, so this costs one matrix axis.
• Don't pin TZ=UTC to get green. The suite passes under UTC, so pinning it would make CI pass while leaving the bug in place for every user east of Greenwich.

That means #71 wants fixing first, or the workflow's first run will be red for a reason unrelated to whatever PR adds it.