Julia/arrowfunc: Arrow function expression

[zhiyuuuu]

1. TS_to_Strata.lean

• Implement translation logic for TS_ArrowFunctionExpression in variable declarations and assignments.
• Register arrow function expressions in the translation context under their bound names.

2. js_ast.lean

• Define the TS_ArrowFunctionExpression structure.
• Extend the TS_Expression inductive type to include TS_ArrowFunctionExpression.

3. conformance_testing/babel_to_lean.py

• Parse Babel "ArrowFunctionExpression" nodes.
• Map them to Lean’s TS_ArrowFunctionExpression structure for conformance testing.