This doesn't follow the exact spec steps but instead simply makes a
nested Core::EventLoop and spins it while a periodic timer tests the
goal condition.
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.
