Intent to ship: ariaNotify

[James Teh]

As of Firefox 150, I intend to turn ariaNotify on by default on all platforms. It has been developed behind the accessibility.ariaNotify.enabled preference. Status in other browsers: shipped in Blink, implemented in WebKit.

Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=2018095

Standard: https://w3c.github.io/aria/#ARIANotifyMixin

This feature was previously discussed in this "Intent to prototype" thread: https://groups.google.com/a/mozilla.org/g/dev-platform/c/7UOkFqbeH7o

TAG Review: https://github.com/w3ctag/design-reviews/issues/1075

Jamie