reset method
Resets navigation to initial state (first item, no scroll).
Implementation
void reset({int initialIndex = 0}) {
if (_itemCount == 0) {
_selectedIndex = 0;
_scrollOffset = 0;
} else {
_selectedIndex = initialIndex.clamp(0, _itemCount - 1);
_scrollOffset = 0;
_adjustScroll();
}
}