LibJS: Implement the GetMethod abstract operation

https://tc39.es/ecma262/#sec-getmethod

We have bunch of duplicated on-demand versions of this, let's do it
properly.

Userland/Libraries/LibJS/Forward.h

@@ -139,6 +139,7 @@ class MarkedValueList;
 class NativeFunction;
 class NativeProperty;
 class PrimitiveString;
+class PropertyName;
 class Reference;
 class ScopeNode;
 class ScopeObject;