feat: mark chat as unread (#475)

Co-authored-by: Pedro S. Lopez <pedroslopez@me.com>

src/structures/Chat.js

@@ -161,6 +161,13 @@ class Chat extends Base {
         return this.client.unmuteChat(this.id._serialized);
     }
 
+    /**
+     * Mark this chat as unread
+     */
+    async markUnread(){
+        return this.client.markChatUnread(this.id._serialized);
+    }
+
     /**
      * Loads chat messages, sorted from earliest to latest.
      * @param {Object} searchOptions Options for searching messages. Right now only limit is supported.