Rework announcements to not use global SHARED object

[pcardune]

Changes:

• SHARED.announcer is gone, as is store.queuedAnnouncements and store.muteAnnouncements
• added mountAnnouncer() function that encapsulates initialization of dom nodes and announcer state
• added private Announcer class that encapsulates state/semantics around the announcement queue
• got rid of muteAnnouncements altogether since it didn't appear to ever be set to anything but false

We're still relying on the correct order of operations to be used in that mountAnnouncer() must be called before say() will do anything, but at least now it's documented. I'm not sure why the dom node that holds the announcements needs to be inside the element that wraps the codemirror editor, but if it doesn't, then we could just add it to document.body instead and we would no longer need mountAnnouncer.

[schanzer]

I'd like to bring back muteAnnouncements. There were some subtle corner cases involving screenreader bug workarounds where our a11y consultant indicated this would be important. Many of those got fixed in the last 2-3 years, so we removed the uses....but I'm not convinced we won't need it again as we enter wider user testing.

[pcardune]

ok, this should do it.