Add end-to-end tests of some sort

[JoshuaKGoldberg]

High unit test coverage makes me feel good on the inside, but doesn't really capture the end-to-end scenario completely. So far I've verified changes work locally by re-running this tool against TypeStat. It'd be good to have some kind of end-to-end tests that verify things like CLI parsing and file writes are as expected.

How should this be done? Jest tests that fork/exec, perhaps?