LibJS: Add the Temporal namespace object :^)

Currently empty, but we gotta start somewhere! This is the start of
implementing the Temporal proposal (currently stage 3).

I have decided to start a new subdirectory (Runtime/Temporal/) as well
as a new C++ namespace (JS::Temporal) for this so we don't have to
prefix all the files and classes with "Temporal" - there will be a lot.

https://tc39.es/proposal-temporal/

Userland/Libraries/LibJS/Runtime/Temporal/Temporal.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Object.h>
+
+namespace JS::Temporal {
+
+class Temporal final : public Object {
+    JS_OBJECT(Temporal, Object);
+
+public:
+    explicit Temporal(GlobalObject&);
+    virtual void initialize(GlobalObject&) override;
+    virtual ~Temporal() override = default;
+};
+
+}