darwinkit: generate struct reference typedefs

[progrium]

Typedefs that the API uses that are not generated include object/struct reference types like these:

https://developer.apple.com/documentation/coregraphics/cgcolorref
https://developer.apple.com/documentation/appkit/nsmodalsession

There aren't many and for now at least the ones referenced in current API coverage are manually added in the <module>_custom.go source file for module/framework packages. These are pretty easy to generate for Go, they're just types based on unsafe.Pointer. But they should live in their own generated source file and be generated based on struct symbols found that are I guess typedef struct aliases to another type with declarations like:

typedef struct CGColor CGColorRef;
typedef struct _NSModalSession NSModalSession;

They usually but don't always have a "Ref" suffix.

[progrium]

So apparently we're right to alias as pointers, but for a lot of these (prob most ending with Ref), we could alias to a type we have bindings for:
https://developer.apple.com/library/archive/documentation/General/Conceptual/CocoaEncyclopedia/Toll-FreeBridgin/Toll-FreeBridgin.html#//apple_ref/doc/uid/TP40010810-CH2