[red-knot] Add fuzzer to catch panics for invalid syntax (#14678)
## Summary
This PR adds a fuzzer harness for red knot that runs the type checker on
source code that contains invalid syntax.
Additionally, this PR also updates the `init-fuzzer.sh` script to
increase the corpus size to:
* Include various crates that includes Python source code
* Use the 3.13 CPython source code
And, remove any non-Python files from the final corpus so that when the
fuzzer tries to minify the corpus, it doesn't produce files that only
contains documentation content as that's just noise.
## Test Plan
Run `./fuzz/init-fuzzer.sh`, say no to the large dataset.
Run the fuzzer with `cargo +night fuzz run red_knot_check_invalid_syntax
-- -timeout=5`
