expandItemKey method
Implementation
void expandItemKey(K key, {Map<String, dynamic>? additional}) {
if (expansionMode == TExpansionMode.none) return;
final newExpandedKeys = expansionMode == TExpansionMode.single ? copyKeySet([key]) : copyKeySet(expandedKeys)
..add(key);
updateExpansionState(newExpandedKeys, additional: additional);
}