Add Scroll Snap events

[chrisdavidmills]

Summary

Chrome 129 supports scroll snap events — scrollsnapchange and scrollsnapchanging, which enable code being fired in response to a new scroll snap target being snapped to, and the browser identifying a change in the next snap target to be snapped to (if the gesture were to end).

See the associated ChromeStatus entry and the Scroll Snap Events blog post for more context and a guide.

This PR adds the new events, and the SnapEvent event object.

Note: The spec defines the associated event handler properties as being defined on the GlobalEventHandlers mixin, but I've added the events to the Element object, as we no longer document GlobalEventHandlers. Let me know if you think it should sit somewhere else.

Test results and supporting details

Related issues

[caugner]

LGTM.

I defer to @queengooborg for whether Element is the right place for the events from the GlobalEventHandlers mixin.

[queengooborg]

For all mixins, we add the members to all the interfaces it applies to!

[chrisdavidmills]

OK, I've done some research and a bit of testing, and it looks like they are supported on Document and Window also. See also the spec mention here: https://drafts.csswg.org/css-scroll-snap-2/#event-handlers-on-elements-document-and-window-objects.