LibJS: Add the CreateMappedArgumentsObject abstract operation

This patch adds a new ArgumentsObject class to represent what the spec
calls "Arguments Exotic Objects"

These are constructed by the new CreateMappedArgumentsObject when the
`arguments` identifier is resolved in a callee context.

The implementation is incomplete and doesn't yet support mapping of
the parameter variables to the indexed properties of `arguments`.

Userland/Libraries/LibJS/Runtime/OrdinaryFunctionObject.h

@@ -41,6 +41,7 @@ protected:
     virtual bool is_strict_mode() const final { return m_is_strict; }
 
 private:
+    virtual bool is_ordinary_function_object() const { return true; }
     virtual FunctionEnvironmentRecord* create_environment_record(FunctionObject&) override;
     virtual void visit_edges(Visitor&) override;