`repeat` alternative that handles animations

[cristinecula]

Handling the in and out animation of HTML elements that are added to the DOM should be moved to a lit directive, similar to the repeat directive. This would decouple the animation from the state and will allow us to do very simple state management operations: setState((notifications) => notifications.filter(...)) or setState((notifications) => notifications.concat(n)). Such a directive would be very useful in many scenarios.

Originally posted by @cristinecula in https://github.com/Neovici/cosmoz-frontend/pull/7832#discussion_r1874410767