feat: mark chat as unread (#475)

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

index.d.ts

@@ -99,6 +99,9 @@ declare namespace WAWebJS {
         /** Mark as seen for the Chat */
         sendSeen(chatId: string): Promise<boolean>
 
+        /** Mark the Chat as unread */
+        markChatUnread(chatId: string): Promise<void>
+
         /** 
          * Sets the current user's status message
          * @param status New status message
@@ -777,6 +780,8 @@ declare namespace WAWebJS {
         unmute: () => Promise<void>,
         /** Returns the Contact that corresponds to this Chat. */
         getContact: () => Promise<Contact>,
+        /** Marks this Chat as unread */
+        markUnread: () => Promise<void>
         /** Returns array of all Labels assigned to this Chat */
         getLabels: () => Promise<Label[]>
     }