Combobox: Include formValue in onSelectionChange callback

## Context

Currently, the `Combobox` component allows you to specify a `formValue` for `ComboboxItem`'s. However, there's not a great way to get the `formValue` of the currently selected item, since it's just rendered as a hidden form value prop.

The current work around is to either map the combobox value back to the list of form values, or to parse the form contents. It would be helpful if the selected combobox item's form value was included in the object passed to the `onSelectionChange` callback: https://github.com/dequelabs/cauldron/blob/6dd5a3404e743f0ec246056c2fe81666e064ce1c/packages/react/src/components/Combobox/Combobox.tsx#L332-L336

## Acceptance Criteria

When the `Combobox` selection changes and the `ComboboxItem` has a `formValue`, the `formValue` (and ideally the "previous" `formValue` for consistency) is provided:

```ts
onSelectionChange?.({
target,
value: stringValue,
previousValue: previousValue?.toString(),
formValue,
previousFormValue
});
```