feat: add more null types for types the runtime accept or returns as null

[edusperoni]

Summary

• Pointer return types now always include | null in generated .d.ts files, matching the runtime behavior where a native null pointer returns JS null (not an interop.Pointer wrapping address 0)
• Pointer parameter types include | null by default, except when the native API is annotated _Nonnull — preserving type safety for APIs that contractually require a non-null pointer
• Adds NonNullableType to the metadata generator's type system to track _Nonnull annotations from clang, mirroring the existing NullableType for _Nullable

Before

// return — no null, but runtime can return null
declare function CFAllocatorAllocate(...): interop.Pointer | interop.Reference<any>;
// param — no null, but runtime accepts null/undefined
declare function fegetenv(p1: interop.Pointer | interop.Reference<fenv_t> | ArrayBufferLike | ArrayBufferView): number;

After

// return — always nullable (runtime returns null for nullptr)
declare function CFAllocatorAllocate(...): interop.Pointer | interop.Reference<any> | null;
// unannotated/nullable param — accepts null
declare function fegetenv(p1: interop.Pointer | interop.Reference<fenv_t> | ArrayBufferLike | ArrayBufferView | null): number;
// _Nonnull param — no null (e.g. NSData.dataWithBytesNoCopyLength)
dataWithBytesNoCopyLength(bytes: interop.Pointer | interop.Reference<any> | ArrayBufferLike | ArrayBufferView, ...): NSData;

Array types (interop.Reference<T> from ConstantArray/ExtVector/IncompleteArray) are not affected — they represent inline data that is never null.

Test plan

• Built metadata generator for arm64
• Generated declarations for 65 modules (27,718 declarations) from the iOS 18.5 SDK
• Verified 20,419 pointer params have | null (nullable/unannotated) and 2,549 do not (_Nonnull)
• Verified all pointer return types include | null
• Verified array types (interop.Reference<T> without interop.Pointer) are unaffected
• Spot-checked real APIs: NSData.dataWithBytesNoCopyLength (nonnull, no null) vs NSData.dataWithBytesLength (nullable, has null), CGBitmapContextCreate(data:) (nullable, has null)