Reach listbox notes

Listbox has turned out to be a real test for us in many ways. Primarily, it
challenges our desire for maximum composability, a key goal for all of the
Reach UI components. A listbox select component essentially consists of:

- A button the user clicks when a listbox is closed
- A list of options in a popover that is displayed after a user clicks

This sounds a lot like MenuButton from a UI perspective, but two key
differences:

- ListboxOption holds a value, whereas a MenuItem does not
- The ListboxButton rendered result depends on the currently selected
  ListboxOption

This last point is the kicker! In order for the ListboxButton to know what's
going on the the ListboxList, we need to update state in context and store it
at the top of the tree. This means we can't show the ListboxButton's inner
content on the first render, which means we can't render ListboxButton on
the server ... UNLESS the component state is controlled in the app.

So in most Reach components, we offer the user the ability to choose between
uncontrolled or controlled state. For an uncontrolled component, all you'd
have to do is compose the parts and everything just works. AWESOME.

We still offer that choice for Listbox, but the concession here is that if
you are server rendering your component you may get a server/client mismatch.
For this reason, if you are server rendering we always recommend using
controlled state for your listbox and explicitly tell the button what to
render at the top of the tree.

In an attempt to mimick the behavior of a native select field, we also realized
quickly that some native select behaviors differ between platforms. For example,
on Windows the Enter key on a closed select will open the select field, but on
a Mac it will stay closed and instead try to submit the associated form,
presuming the select is inside of a form. To keep the code lean and release the
component more quickly, we decided to use the general MacOS behavior as a guide.
We would like to detect the user's OS and adjust the behavior accordingly, but
we want to be deliberate and careful with our approach to doing so.