LibJS: Add getter/setter support

This patch adds a GetterSetterPair object. Values can now store pointers
to objects of this type. These objects are created when using
Object.defineProperty and providing an accessor descriptor.

Libraries/LibJS/Forward.h

@@ -59,6 +59,7 @@ class DeferGC;
 class Error;
 class Exception;
 class Expression;
+class Accessor;
 class GlobalObject;
 class HandleImpl;
 class Heap;