Support `scrollend` event

[nicoburns]

We should implement the scrollend events which fires when a page has finished scrolling.

The scrollend event fires when:

• The browser is no longer animating or translating scroll.
• The user's touch has been released
• The user's pointer has released the scroll thumb.
• The user's keypress has been released
• Scroll to fragment has completed
• Scroll snap has completed
• scrollTo() has completed
• The user has scrolled the visual viewport.

The scrollend event does not fire when:

• A user's gesture did not result in any scroll positional changes (no translation occurred).
• scrollTo() did not result in any translation.

https://developer.chrome.com/blog/scrollend-a-new-javascript-event#event_details