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

---------

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

index.d.ts

@@ -223,6 +223,12 @@ declare namespace WAWebJS {
             /** The new ACK value */
             ack: MessageAck
         ) => void): this
+        
+        /** Emitted when a chat unread count changes */
+        on(event: 'unread_count', listener: (
+            /** The chat that was affected */
+            chat: Chat
+        ) => void): this
 
         /** Emitted when a new message is created, which may include the current user's own messages */
         on(event: 'message_create', listener: (