react: ensure we don't show old errors in typing indicators

[vladvelici]

Context

I've noticed that this error was shown briefly when loading the demo app and started digging:

This only happens when you are still waiting for something from a room that's been released, which means we update an old state. The room is only released when a parent component of what we see there (ChatRoomProvider) is unmounted.

The existing version of this code was also not setting the unsubscribe from the hook correctly.

This version uses a simple mounted variable to address the problem. Commits in a [previous PR #342] (https://github.com/ably/ably-chat-js/pull/342) attempted 2 different approaches but both turned out be more trouble than worth.

Checklist

• [x] QA'd by the author.
• [ ] Unit tests created (if applicable).
• [ ] Integration tests created (if applicable).
• [ ] Follow coding style guidelines found here.
• [x] TypeDoc updated (if applicable).
• [ ] (Optional) Update documentation for new features.
• [ ] Browser tests created (if applicable).
• [x] In repo demo app updated (if applicable).

Testing Instructions (Optional)

Open demo app - enjoy the absence of the flashing error.

[github-actions[bot]]

Coverage Report

Status	Category	Percentage	Covered / Total
🟢	Lines	94.5% (🎯 92%)	2839 / 3004
🟢	Statements	94.5% (🎯 92%)	2839 / 3004
🟢	Functions	96.36% (🎯 93%)	212 / 220
🟢	Branches	94.47% (🎯 93%)	770 / 815

File Coverage

File	Stmts	% Branch	% Funcs	% Lines	Uncovered Lines
Changed Files
src/react/hooks/use-typing.ts	98.83%	93.1%	100%	98.83%	85

Generated in workflow #1497