LibJS: Begin implementing Atomics

This adds the Atomics object to the global object and sets up only its
@@toStringTag property.

Userland/Libraries/LibJS/Runtime/AtomicsObject.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Object.h>
+
+namespace JS {
+
+class AtomicsObject : public Object {
+    JS_OBJECT(AtomicsObject, Object);
+
+public:
+    explicit AtomicsObject(GlobalObject&);
+    virtual void initialize(GlobalObject&) override;
+    virtual ~AtomicsObject() override = default;
+};
+
+}