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/ArgumentsObject.cpp

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/ArgumentsObject.h>
+#include <LibJS/Runtime/GlobalObject.h>
+
+namespace JS {
+
+ArgumentsObject::ArgumentsObject(GlobalObject& global_object)
+    : Object(*global_object.object_prototype())
+{
+}
+
+void ArgumentsObject::initialize(GlobalObject& global_object)
+{
+    Base::initialize(global_object);
+}
+
+ArgumentsObject::~ArgumentsObject()
+{
+}
+
+}