Called when a FormItem's value is about to change as the result of user interaction. This method fires after the user performed an
action that would change the value of this field, and allows the developer to modify / reformat the value before it gets validated / saved.
Fires before the change event.
form - the managing DynamicForm instane
item - the form item itself
value - the new value of the form item
oldValue - the previous (current) value of the form item