update method
Implementation
void update(Date? visibleMonth, Date minDate, Date maxDate) {
if (visibleMonth == null) return;
hasPrev.value = compareDatesAtResolution(
visibleMonth, minDate, CalendarResolution.months) >
0;
hasNext.value = compareDatesAtResolution(
visibleMonth, maxDate, CalendarResolution.months) <
0;
}