Show timestamp on Notification UI

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> Sometimes notifications can be stale, or multiple actions can show notifications quickly, such that it would be helpful to have a timestamp displayed on each notification. I think it would be fine for these to be "dumb" and just local to the user, but of course I may be missing nuances.