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/EventLoop.cpp

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/VM.h>
+#include <LibWeb/Bindings/MainThreadVM.h>
+#include <LibWeb/HTML/EventLoop/EventLoop.h>
+
+namespace Web::HTML {
+
+EventLoop::EventLoop()
+{
+}
+
+EventLoop::~EventLoop()
+{
+}
+
+EventLoop& main_thread_event_loop()
+{
+    return static_cast<Bindings::WebEngineCustomData*>(Bindings::main_thread_vm().custom_data())->event_loop;
+}
+
+}