LibJS: Start implementing ShadowRealm

This commit adds the ShadowRealm object itself, its constructor, and
prototype (currently empty).

Userland/Libraries/LibJS/Runtime/ShadowRealmConstructor.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/NativeFunction.h>
+
+namespace JS {
+
+class ShadowRealmConstructor final : public NativeFunction {
+    JS_OBJECT(ShadowRealmConstructor, NativeFunction);
+
+public:
+    explicit ShadowRealmConstructor(GlobalObject&);
+    virtual void initialize(GlobalObject&) override;
+    virtual ~ShadowRealmConstructor() override = default;
+
+    virtual Value call() override;
+    virtual Value construct(FunctionObject& new_target) override;
+
+private:
+    virtual bool has_constructor() const override { return true; }
+};
+
+}