addListener method
Inserts data
at the end of the list.
Implementation
void addListener(MiniSubscription<T> data) {
var newNode = Node(data: data);
if (isEmpty) {
_head = newNode;
} else {
var currentNode = _head!;
while (currentNode.next != null) {
currentNode = currentNode.next!;
}
currentNode.next = newNode;
}
}