SpinButton: add example for input onChange/onBlur behavior

[spmonahan]

Problem

SpinButton needs a demo showing how a user can listen for onChange/onBlur (or really any event) on the input. By default SpinButton does not dispatch a change event for every keystroke on its input only on value commits (enter press for example). This is the cut down on "noise" in the onChange event listener.

Solution

Users can listen for every keystroke by attaching an onChange listener to the input slot.

[spmonahan]

Codesandbox example: https://codesandbox.io/s/react-spinbutton-slots-onblur-and-onchange-k9nrri?file=/src/App.tsx