fix(parser): parse default async arrow exports

[AayushMainali-Github]

Fixes #5442.

Summary

• distinguish async function declarations from async arrow expressions after export default
• parse async arrow functions through the default assignment-expression path
• add regression coverage for parenthesized, unparenthesized, and multi-parameter forms

Testing

• cargo test -p boa_parser
• cargo clippy -p boa_parser --all-targets -- -D warnings
• cargo fmt --all -- --check