Announce newest notification using live region

[engijlr]

Summary:
Add one always-present live region and, on each new notification, replace its text so screen readers announce it. I left the styles bare on purpose so it’s easier to test. I’ll tidy them up next.

Things I considered:

• The alert doesn’t need focus, it announces automatically.
• The message must be added dynamically, not only on initial page load.
• The element with role="alert" must exist in the DOM first.
• Don’t create a new role="alert" element that’s already filled with text; update an existing element’s text instead.

Changes:

• Added #sr-live (role="alert") to the container.
• On notifications update, announce the latest item.
• Added _announce(text) to update the live region.
• Added _getNotificationText(...) to read the message from the handler’s data.

How to test:
Enable a screen reader(Try Safari; it works better in Safari, I don't know why.)
Trigger a toast (Perform any action that shows a notification).
Expected: VoiceOver announces the notification text once the toast appears.

Possible improvements:
-Choose assertive vs. polite based on notification.color.
-Move the screen-reader announcement logic for toasts into the UUI library.

[nielslyngsoe]

Looks good