Don't send typing notifications unless `send_private_typing_notifications` setting is enabled.

[timabbott]

In zulip/zulip#19200, we've added new privacy settings for whether to send typing notifications.

The server will reject typing notification requests if this setting is disabled, so it won't be a user-facing bug to ignore this unless the client-side error handling causes that to be the case. I'll be deploying the feature to CZO soon.

(Note that we're also adding send_stream_typing_notifications, but stream-level typing notifications are not fully implemented yet -- though I think it's mostly client code that's missing and the server implementation is complete). See #1015 for details on that.