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.h

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Object.h>
+
+namespace JS {
+
+class ArgumentsObject final : public Object {
+    JS_OBJECT(ArgumentsObject, Object);
+
+public:
+    explicit ArgumentsObject(GlobalObject&);
+
+    virtual void initialize(GlobalObject&) override;
+    virtual ~ArgumentsObject() override;
+};
+
+}