Add to read-receipt iface

src/models/read-receipt.ts

@@ -309,4 +309,13 @@ export abstract class ReadReceipt<
         // We don't know if the user has read it, so assume not.
         return false;
     }
+
+    /**
+     * Returns the most recent unthreaded receipt for a given user
+     * @param userId - the MxID of the User
+     * @returns an unthreaded Receipt. Can be undefined if receipts have been disabled
+     * or a user chooses to use private read receipts (or we have simply not received
+     * a receipt from this user yet).
+     */
+    public abstract getLastUnthreadedReceiptFor(userId: string): Receipt | undefined;
 }