Add `objc_registerClassPair` to allow subclassing

vendor/darwin/Foundation/objc.odin

@@ -11,6 +11,7 @@ foreign Foundation {
 	objc_lookUpClass       :: proc "c" (name: cstring) -> Class ---
 	sel_registerName       :: proc "c" (name: cstring) -> SEL ---
     objc_allocateClassPair :: proc "c" (superclass : Class, name : cstring, extraBytes : c.size_t) -> Class ---
+	objc_registerClassPair :: proc "c" (cls : Class) ---
 
 	class_addMethod :: proc "c" (cls: Class, name: SEL, imp: IMP, types: cstring) -> BOOL ---
 	class_getInstanceMethod :: proc "c" (cls: Class, name: SEL) -> Method ---