addItem method
Adds a single item. If index is provided, inserts at that position.
Implementation
void addItem(M3EDropdownItem<T> item, {int index = -1}) {
if (index == -1) {
_items.add(item);
} else {
_items.insert(index, item);
}
_reapplySearchFilter();
notifyListeners();
onSelectionChange?.call(selectedItems);
}