feat: handle notification.mark_unread

[MartinCupela]

Description of the changes, What, Why and How?

Upon receiving the new notification.mark_unread event update:

• channel read state for "author" of the event
• update channel's unreadCount property

A new property added to the ChannelReadStatus is first_unread_message_id which basically is an indicator whether the channel has been marked unread. The property is undefined for message.new events.

With the above state updates, the SDK can implement the feature of marking messages in a channel as unread.

[github-actions]

Size Change: +607 B (0%)

Total Size: 328 kB

Filename	Size	Change
dist/browser.es.js	70.6 kB	+95 B (0%)
dist/browser.full-bundle.min.js	43.9 kB	+232 B (+1%)
dist/browser.js	71.6 kB	+90 B (0%)
dist/index.es.js	70.6 kB	+100 B (0%)
dist/index.js	71.6 kB	+90 B (0%)

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