Timeline live region includes undesireable things

[turt2live]

Spun off from #9747 and #302

[turt2live]

see matrix-org/matrix-react-sdk#3019 and co for bad implementation

[t3chguy]

Such as

• duplicates when you are sending messages due to the sending->sent->null status change
• history as you backfill it/scroll up

The Live Region should only include the fragment of the timeline which is not sending/sent and also added on the forward end since first render.

One way to visualise this is splitting the timeline into 3 regions:

• Historical fragment: the entire timeline at the time of rendering (status === null)
• Live region fragment: the new events which have entered if we are at the live end of the timeline (status === null)
• Pending fragment: any events which are in the pending timeline (status !== null)

Bonus points:
upon sending out own message a screen reader should just say "Message sent" rather than reading the entire thing out again