Add the 'unread_count' event. Listen to changes on Chat.unreadCount (#2059)

* Initial commit

* Update index.d.ts

* The functions was returning a 'brief' chat data. With this, it will return full Chat model..

* Eslint suggested fixex

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Co-authored-by: tuyuribr <45042245+tuyuribr@users.noreply.github.com>
Co-authored-by: Rajeh Taher <rajeh@reforward.dev>

src/util/Constants.js

@@ -41,6 +41,7 @@ exports.Events = {
     MESSAGE_REVOKED_EVERYONE: 'message_revoke_everyone',
     MESSAGE_REVOKED_ME: 'message_revoke_me',
     MESSAGE_ACK: 'message_ack',
+    UNREAD_COUNT: 'unread_count',
     MESSAGE_REACTION: 'message_reaction',
     MEDIA_UPLOADED: 'media_uploaded',
     GROUP_JOIN: 'group_join',