[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`

.github/workflows/ci.yaml

@@ -32,6 +32,8 @@ jobs:
       # Flag that is raised when any code is changed
       # This is superset of the linter and formatter
       code: ${{ steps.changed.outputs.code_any_changed }}
+      # Flag that is raised when any code that affects the fuzzer is changed
+      fuzz: ${{ steps.changed.outputs.fuzz_any_changed }}
     steps:
       - uses: actions/checkout@v4
         with:
@@ -79,6 +81,11 @@ jobs:
               - python/**
               - .github/workflows/ci.yaml
 
+            fuzz:
+              - fuzz/Cargo.toml
+              - fuzz/Cargo.lock
+              - fuzz/fuzz_targets/**
+
             code:
               - "**/*"
               - "!**/*.md"
@@ -288,7 +295,7 @@ jobs:
     name: "cargo fuzz build"
     runs-on: ubuntu-latest
     needs: determine_changes
-    if: ${{ github.ref == 'refs/heads/main' }}
+    if: ${{ github.ref == 'refs/heads/main' || needs.determine_changes.outputs.fuzz == 'true' }}
     timeout-minutes: 10
     steps:
       - uses: actions/checkout@v4