Add f-string preprocessing

[aspizu]

Summary

• add f-string preprocessing with expression interpolation via join tokens
• support escaped braces with {{ and }}
• stop parsing after preprocessing errors to avoid follow-on diagnostics
• call the local tsx binary when validating sb3 output

Tests

• cargo +nightly test pre_processor
• tools/run.py -v
• cargo +nightly clippy --all-targets --all-features