fix(index): export NotificationKind (#4514)

Closes #4513

Author: cartant

src/index.ts

@@ -24,7 +24,7 @@ export { Subscription } from './internal/Subscription';
 export { Subscriber } from './internal/Subscriber';
 
 /* Notification */
-export { Notification } from './internal/Notification';
+export { Notification, NotificationKind } from './internal/Notification';
 
 /* Utils */
 export { pipe } from './internal/util/pipe';