fix(parser): support async arrow functions in default exports

[AdityaOP007]

Summary

#5442

This PR fixes a parser issue where valid ECMAScript syntax using an async arrow function as a default export was incorrectly rejected.

Previously, statements like:

export default async (x) => x;

resulted in a parser error because export default async always assumed an async function declaration instead of allowing an AssignmentExpression. According to the ECMAScript specification, an async arrow function is a valid AssignmentExpression and should parse successfully.

What Changed

• Updated the default export parser to apply the correct ECMAScript lookahead for async.
• Only parses a hoistable async function when async is followed by function (with no line terminator).
• Falls back to parsing an AssignmentExpression in all other cases, allowing async arrow functions to be handled by the existing parser logic.
• Added parser tests covering valid async arrow default exports, invalid syntax, regression scenarios, and AST validation to ensure the generated syntax tree is correct.

Result

The following syntax now parses successfully:

export default async () => 1;
export default async x => x;
export default async (x) => x;
export default async (a, b) => a + b;

Existing valid forms such as:

export default async function () {}
export const fn = async (x) => x;

continue to work as expected.

Validation

• ✅ cargo fmt
• ✅ cargo clippy -- -D warnings
• ✅ Parser test suite passed
• ✅ Workspace tests passed

This change brings the parser behavior in line with the ECMAScript specification while preserving existing functionality and adding regression coverage to prevent similar issues in the future.