addListener method
Adds a listener to the adapter.
Implementation
@override
void addListener(FocusNode node, NesInputEvent event, VoidCallback callback) {
if (!_events.containsKey(node)) {
_events[node] = {};
}
_events[node]?.putIfAbsent(event, () => List.empty(growable: true));
_events[node]?[event]?.add(callback);
}