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.cpp

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/HTML/EventLoop/Task.h>
+
+namespace Web::HTML {
+
+Task::Task(DOM::Document* document, Function<void()> steps)
+    : m_steps(move(steps))
+    , m_document(document)
+{
+}
+
+Task::~Task()
+{
+}
+
+void Task::execute()
+{
+    m_steps();
+}
+
+}