feat(account): add event for deleting read items in notification center

- Add new event class `InAppNotificationCenterReadItemsDeleted`
- This event is dispatched when user requests to delete all read items in the active tab

Author: fulleni

lib/account/bloc/in_app_notification_center_event.dart

@@ -62,3 +62,10 @@ class InAppNotificationCenterFetchMoreRequested
     extends InAppNotificationCenterEvent {
   const InAppNotificationCenterFetchMoreRequested();
 }
+
+/// Dispatched when the user requests to delete all read items in the
+/// currently active tab.
+class InAppNotificationCenterReadItemsDeleted
+    extends InAppNotificationCenterEvent {
+  const InAppNotificationCenterReadItemsDeleted();
+}