Add TransitionChild/Root components

Currently this package only provides `<Transition>` and `<TransitionGroup>` components that apply styles to direct individual elements.
But you cannot easily toggle exit/enter classes in nested elements.
Is it something solved by `TransitionChild/Root` components in react/vue.
Here is an example how it could be used:
https://tailwindui.com/components/application-ui/overlays/slide-overs