Add unit tests

[scottredig]

This project could really use some unit tests. Unfortunately, all of the important behavior would also need a JS environment to actually test. This is an area where I'd be very happy to accept contributions, given that I'm unlikely to do it for myself.

I haven't done any research into what JS environment would be easiest to get working with the Zig build system. A strong requirement here is that zig build test (possibly performing some zon fetches) must work without any other external dependencies. Cross platform is a bonus, but I'm on windows so that's a requirement.