LibJS: Make NativeProperty a plain Cell instead of an Object

This removes the need for NativeProperty objects to have a prototype,
which just made things confusing.

Libraries/LibJS/Forward.h

@@ -89,6 +89,7 @@ class HeapBlock;
 class Interpreter;
 class LexicalEnvironment;
 class MarkedValueList;
+class NativeProperty;
 class PrimitiveString;
 class Reference;
 class ScopeNode;