add ffi_util package for FFI related util & raw function type

[Guest0x0]

When binding FFI in wasm/native backend, it is often necessary to pass a callback function to FFI, and sometimes the FFI function must be capture free, for example Unix signal handler.

This PR adds a new package @ffi_util and a new type @ffi_util.RawFunction[T]. @ffi_util.RawFunction[T] represents capture-free toplevel function of type T. Note that the definition in core is merely a placeholder, the raw function type need special treatment from the compiler. So it is not usable until the compiler part is released.

[peter-jerry-ye-code-review]

‼️ This code review is generated by a bot. Please verify the content before trusting it.

⚠️ [Conflicting Type Definitions for RawFunction]

• Category: Correctness
• Code Snippet:
  ffi_util.mbti (line 4): pub type RawFunction
raw_function.mbt (line 4): pub type RawFunction[T]
• Recommendation: Remove the non-generic RawFunction declaration from ffi_util.mbti and keep only the generic RawFunction[T] in raw_function.mbt
• Reasoning: The compiler will treat these as conflicting type declarations since they share the same name but have different type parameters. This would prevent the package from compiling correctly.

💡 [Incomplete Documentation for Specialized Type]

• Category: Maintainability
• Code Snippet: raw_function.mbt documentation comment
• Recommendation: Expand documentation to include:

  /// Example: `let handler: RawFunction[(Int) -> Unit] = fn(...) {...}`
  /// Compiler will verify the function has no captured variables
  /// Only works with toplevel function definitions
  

• Reasoning: The current documentation doesn't clarify usage patterns or enforcement mechanism for capture-free requirement, which is critical for a compiler-magic type.

⚠️ [Potential Package Structure Issue]

• Category: Maintainability
• Code Snippet: moon.pkg.json being empty
• Recommendation: Add minimal package metadata like {"name": "ffi_util"}
• Reasoning: While MoonBit might tolerate empty package files, explicit metadata improves discoverability and prevents potential tooling issues.

[bobzhang]

per offline discussion, let's make it builtin type