parser: recognize exponentiation operators

[Photon101]

Summary

• Map the lexer Circumflex token to BinaryOperator::Raise, so both ^ and ** parse as exponentiation.
• Keep % at multiplicative precedence instead of exponentiation precedence.
• Add parser regression coverage for exponentiation precedence, right associativity, and mixed */% expressions.

Fixes #35.

Checks

All checks were run with Rust/Cargo stable 1.96.0.

• cargo test -p parser test_parser_exponentiation
• cargo test -p parser test_parser_multiplicative_precedence
• cargo test -p parser
• cargo fmt --all -- --check
• cargo check --workspace
• cargo build --workspace
• cargo test --workspace
• cargo clippy --workspace -- -D warnings
• cargo check --target wasm32-wasip1
• git diff --check

[Alonely0]

Simple and effective, tysm! We could use some more hands, so we'd appreciate you sticking around ;)