Implement some form of automated testing

[invicticide]

As the codebase grows in complexity -- and if we start getting more contributors over time -- it's going to become ever more important to have some kind of test suite to ensure stability and avoid bug regressions.

So far we've been informally using the examples projects and git diffs to identify any weirdness, but we're starting to outgrow that.

I'd like to figure out some reasonable automated testing approach that's not super burdensome but does provide good test coverage and requires little-to-no manual intervention. Optimally it would be something we could hook up to npm run test and thereby run the test battery automatically in front of any deploy/version/publish request.