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/Runtime/Shape.h

@@ -40,6 +40,8 @@ struct Attribute {
         Configurable = 1 << 0,
         Enumerable = 1 << 1,
         Writable = 1 << 2,
+        HasGet = 1 << 3,
+        HasSet = 1 << 4,
     };
 };