Add a unit test suite

[toolness]

I normally start a test suite whenever I create a new project, but I didn't do that for a few reasons here:

• I'd have to figure out how to wrangle the build system to run a test suite, which would take additional time.
• Thanks to TypeScript, my code felt a lot less fragile than it normally does.
• The widget documentation actually doubles as a decent manual test suite.

That said, a foundation for building unit tests would still be helpful.