Adds an event handler that runs when a value of the current item changes.
The onItemValuesChanged() function allows you to optionally perform actions
right after the current item's values change. The item's value changes
when a user changes the value in one of the item's connected page elements
or you change the value programmatically.
Calling onItemValuesChanged() on a read-only dataset causes an error.
handler(itemBeforeChange: object, updatedItem: object): void The current value changed event handler.