Add TransitionChild/Root components

[thetarnav]

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