The environment settings object is effectively the context a piece of
script is running under, for example, it contains the origin,
responsible document, realm, global object and event loop for the
current context. This effectively replaces ScriptExecutionContext, but
it cannot be removed in this commit as EventTarget still depends on it.
https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object
Since we can't simply give HTML::EventLoop control of the whole program,
we have to integrate with Core::EventLoop.
We do this by having a single-shot 0ms Core::Timer that we start when
a task is added to the queue, and restart after processing the queue and
there are still tasks in the 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.
