Add support for progress events

[rdmurphy]

This should be an opt-in feature that is expected to be flagged on every instance of scroll-scene that wants them to activate.

<scroll-scene progress></scroll-scene>
<!-- may be better to use data attributes -->
<scroll-scene data-progress></scroll-scene>

This flag would cause an instance of scroll-scene to interface with the global observer and set up a scroll listener for calculating scroll depth for that element once it enters the view and removes the listener when it exits the view.

The user would still be expected to set up their own listener for this event to react to it.

document.addEventListener('scroll-scene-progress', ({ detail }) => {
  const element = detail.element;
 const progress = detail.progress;
  // do what you need to do
});

[rdmurphy]

Added via 1f61d39.