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/Interpreter.h

@@ -192,6 +192,10 @@ public:
 
     String join_arguments() const;
 
+    Value resolve_this_binding() const;
+    const LexicalEnvironment* get_this_environment() const;
+    Value get_new_target() const;
+
 private:
     Interpreter();