LibWeb: Move Timer from DOM directory & namespace to HTML

Timers are part of the HTML spec. :^)
https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#timers

Userland/Libraries/LibWeb/HTML/Timer.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+#include <AK/Function.h>
+#include <LibCore/Forward.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::HTML {
+
+class Timer final : public RefCounted<Timer> {
+public:
+    static NonnullRefPtr<Timer> create(Window& window, i32 milliseconds, Function<void()> callback, i32 id);
+    ~Timer();
+
+    void start();
+
+private:
+    Timer(Window& window, i32 milliseconds, Function<void()> callback, i32 id);
+
+    RefPtr<Core::Timer> m_timer;
+    Window& m_window;
+    i32 m_id { 0 };
+};
+
+}