selectWhere method
selects the items that satisfy the predicate.
The predicate parameter is a function that takes a DropdownItem and returns a boolean.
Implementation
void selectWhere(bool Function(DropdownItem<T> item) predicate) {
_items.where(predicate).forEach((element) {
_selected.add(element);
element.selected = true;
});
notifyListeners();
_onSelectionChanged?.call(_selectedValues);
}