Reject unsupported let/const declarations explicitly

[memburg]

Problem

let x = 1; and const x = 1; currently fail through generic invalid-assignment errors. Since GiavaScript intentionally supports var declarations only, these unsupported declaration forms should have explicit behavior and regression coverage.

Scope

• Add tests for unsupported let declarations.
• Add tests for unsupported const declarations.
• Return a clear intentional error instead of a generic assignment-target error.
• Keep var as the only supported declaration form.

Acceptance Criteria

• let x = 1; fails with an intentional unsupported-declaration error.
• const x = 1; fails with an intentional unsupported-declaration error.
• Existing var declaration behavior remains unchanged.
• crystal spec passes.