* react-spinbutton: widen `value` type to include `null`

This commit widens the `value` (and `defaultValue`) type from `number`
to `number | null`. This change is based on partner feedback about
wanting a value to represent "no value".

`null` was chosen because Typescript can easily ensure users handle the
null case as opposed to a value like `NaN`. `null` is also obviously a
special case so it's clearer that it should be handled specially.
Because `null` is a special, non-`number` value it can more simply be
handled internally by SpinButton.

`undefined` cannot be used
as React will interpret `value`/`defaultValue` toggling from a number
to undefined as the component switching from controlled to uncontrolled
(which is logged as an error).

Users could choose an "unset" value that
is also a number like `-1` but that only works if the "unset" value is
outside the value SpinButton range (and won't work if there is no
[min, max] range) and requires some special casing logic.

* Update packages/react-components/react-spinbutton/src/components/SpinButton/useSpinButton.tsx

Co-authored-by: Makoto Morimoto <Humberto.Morimoto@microsoft.com>

* Update packages/react-components/react-spinbutton/src/components/SpinButton/SpinButton.types.ts

Co-authored-by: Makoto Morimoto <Humberto.Morimoto@microsoft.com>

* react-spinbutton: update `null` onchange behavior

Previous the `onChange` event was special cased for `null`, firing
any time a user attempted to change the value even though SpinButton's
implementation will never change a null value internally (it can only be
changed by setting props).

This change updates the `null` `onChange` behavior to be the same as any
other value. `onChange` no longer fire when a user attempts to change from
this value.

* react-spinbutton: improve onchange test assertions

Updates SpinButton tests to include assertions for the number of times
onChange is called.

* update API snapshot

* react-spinbutton: handle steps from `null`

Based on partner feedback the behavior of setting value/defaultValue to
`null` has been updated.

This updated behavior mirrors native HTML spinbutton (input type="number")
behavior when `value=""`.

Prior to this change if `value={null}` it was not possible for a user to
step SpinButton's value using the step buttons. Further, because onChange
only fires when SpinButton's value changes, and it cannot be changed
when it's null (null + a number doesn't make sense), applications would
have no way to know a user was attempting to change the value.

This commit updates `value/defaultValue={null}` such that when a user
clicks a step button:

1. SpinButton assumes a value of "0" is `min` is undefined, otherwise
   a value of `min` is assumed.
2. Steps from the value selected in step 1
3. Clamps the value
4. Commits the value

An uncontrolled SpinButton will behave like a native HTML spinbutton.
A controlled SpinButton requires the application to take action to
update the value so the application can decide what to do when a
user clicks in this state.

* react-spinbutton: update null handling of aria-valuenow

Co-authored-by: Makoto Morimoto <Humberto.Morimoto@microsoft.com>