[AriaNotify] IA2 Support

[alisonmaher]

As discussed in #1958 (comment), we plan to support ariaNotifyon Windows solely through UIA. Some questions were raised around whether we should support this for IA2, as well.

The most widely used ATs already listen to UIA Notification events. If we fallback to live regions in the case of IA2, how do we best handle cases where ATs, like JAWs and NVDA, listen to both mechanisms on Windows? This would require some way to avoid duplicate events.

Some had also raised the question around how this would work for browsers that don’t support UIA. Chromium is in the process of experimenting with UIA. And Firefox is currently implementing UIA.

As such, my proposal is to only plumb ariaNotify through the UIA Notification event on Windows, and IA2 will be ignored. This will prevent duplicate events being fired on ATs, like JAWS and NVDA.