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/Bindings/MainThreadVM.h

@@ -7,9 +7,16 @@
 #pragma once
 
 #include <LibJS/Forward.h>
+#include <LibWeb/HTML/EventLoop/EventLoop.h>
 
 namespace Web::Bindings {
 
+struct WebEngineCustomData final : public JS::VM::CustomData {
+    virtual ~WebEngineCustomData() override { }
+
+    HTML::EventLoop event_loop;
+};
+
 JS::VM& main_thread_vm();
 
 }