moveItem method
Reparents itemKey under toSection. When index is null, the
item is appended to toSection's children.
No animate parameter: the underlying TreeController.moveNode
is a repositioning op, not an insert/remove; nothing animates.
Implementation
void moveItem(K itemKey, {required K toSection, int? index}) {
_checkNotDisposed();
_requireItem(itemKey, "moveItem");
_requireSection(toSection, "moveItem(toSection)");
_tree.moveNode(ItemKey<K>(itemKey), SectionKey<K>(toSection), index: index);
}