This PR does away with the extra type parameters on Div and instead
introduces two wrapper elements, `Stateful` and `Focusable`. All of the
interactivity is stored on `Interactivity` and `InteractiveState`, which
is stored on the base element. The wrappers simply control what methods
are available to call.

Not sure this is fully working, but a smoke test does work.

/cc @as-cii @ConradIrwin 

Release Notes:

- N/A