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/SymbolConstructor.cpp

@@ -76,7 +76,7 @@ Value SymbolConstructor::call(Interpreter& interpreter)
 
 Value SymbolConstructor::construct(Interpreter& interpreter)
 {
-    interpreter.throw_exception<TypeError>(ErrorType::NotACtor, "Symbol");
+    interpreter.throw_exception<TypeError>(ErrorType::NotAConstructor, "Symbol");
     return {};
 }