Implement reflection support for function pointer types and add tests

- Implement handling of FnPtr TypeKind in const-eval, including:
  - Unsafety flag (safe vs unsafe fn)
  - ABI variants (Rust, Named(C), Named(custom))
  - Input and output types
  - Variadic function pointers
- Add const-eval tests covering:
  - Basic Rust fn() pointers
  - Unsafe fn() pointers
  - Extern C and custom ABI pointers
  - Functions with multiple inputs and output types
  - Variadic functions
- Use const TypeId checks to verify correctness of inputs, outputs, and payloads