Beginnings of typed accessors for client data and outputs

[jcheng5]

This will be necessary for plotly automatic resizing, I believe

[wch]

Instead of implementing individual functions, it might be better to implement a __getattr__ method. That would provide a place to handle outputs that don't exist yet.

Another useful thing with a __getattr__ method is that it could return reactive.Value object itself, instead of only providing a way to invoke () on the object. Although... that could also be accomplished @property decorator -- and using that decorator on each individual item would also allow more specific typing, like Value[bool].

[jcheng5]

I don't like the __getattr__ due to the inability to type or document each value (right?). The @property could work though. I don't love that reactive.Value exposes .set() that doesn't actually work, but the other methods on Value could be helpful.