LibJS: Initial class implementation; allow super expressions in object

literal methods; add EnvrionmentRecord fields and methods to
LexicalEnvironment

Adding EnvrionmentRecord's fields and methods lets us throw an exception
when |this| is not initialized, which occurs when the super constructor
in a derived class has not yet been called, or when |this| has already
been initialized (the super constructor was already called).

Libraries/LibJS/Runtime/NativeFunction.cpp

@@ -68,4 +68,9 @@ Value NativeFunction::construct(Interpreter&)
     return {};
 }
 
+LexicalEnvironment* NativeFunction::create_environment()
+{
+    return interpreter().heap().allocate<LexicalEnvironment>(global_object(), LexicalEnvironment::EnvironmentRecordType::Function);
+}
+
 }