LibWeb: Add a bare-bones HTML event loop with a task queue

This patch attaches a HTML::EventLoop to the main thread JS::VM used
for JavaScript bindings in the web engine.

The goal here is to model the various task scheduling mechanisms of the
HTML specification.

Userland/Libraries/LibWeb/HTML/EventLoop/Task.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Function.h>
+#include <AK/NonnullOwnPtr.h>
+#include <AK/RefPtr.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::HTML {
+
+class Task {
+public:
+    static NonnullOwnPtr<Task> create(DOM::Document* document, Function<void()> steps)
+    {
+        return adopt_own(*new Task(document, move(steps)));
+    }
+    ~Task();
+
+    void execute();
+
+    DOM::Document* document() { return m_document; }
+    DOM::Document const* document() const { return m_document; }
+
+private:
+    Task(DOM::Document*, Function<void()> steps);
+
+    Function<void()> m_steps;
+    RefPtr<DOM::Document> m_document;
+};
+
+}