Fuzz testing for Pyrefly

# Summary

It might be reasonable to add [fuzz testing](https://en.wikipedia.org/wiki/Fuzzing) to the project to catch non-obvious bugs. It would be especially useful for catching panics.

I can work on this, but first I'd like your opinion. Would it be a good fit for Pyrefly?

# Popular Tools
- [cargo-fuzz](https://github.com/rust-fuzz/cargo-fuzz)
- [afl.rs](https://github.com/rust-fuzz/afl.rs)
- [honggfuzz-rs](https://github.com/rust-fuzz/honggfuzz-rs)

# References
- [Ruff fuzz](https://github.com/astral-sh/ruff/tree/main/fuzz)
- [Regex fuzz](https://github.com/rust-lang/regex/tree/master/fuzz)
- [Rust-url fuzz](https://github.com/servo/rust-url/blob/43f47e2fcfdd132c531fb05aa16171ca85be95f4/url/fuzz/fuzz_targets/fuzz-url.rs)