Add type checker to verify type annotations

[rzuckerm]

There are lots of them:

• mypy - Older, can be sometimes flaky and very slow
• ty - Fast, Rust-based. Made by Astral
• pyright - Used by Microsoft for their Pylance VSCode extension
• pyrefly - Used by Meta

I have the most experience with mypy, and I'm not a huge fan. I'd probably pick ty, which is Astral who make uv and ruff.