Enable type checking for tests

[rekmarks]

Vitest currently does not check types in tests or source code; it just elides them and runs the raw JavaScript. Almost the entire team relies on their IDEs to catch these errors, but it would be great if our tooling catches it as well. At the very list, errors like the one exemplified by #179 should not be permitted on our default branch.

Vitest has facilities for type checking that we should investigate. Note that this may prevent us from using certain other features, although this could be well worth the cost.

Potentially related: #72

[rekmarks]

We should definitely do this via Vitest. Other org monorepos use Jest with the ts-jest preset, which statically checks types without building before .ts tests are run.