pause method
Implementation
Future<void> pause(String id) async {
final UDownloadTask? task = this.task(id);
if (task == null || !task.canPause || task.system) return;
final _Runner? runner = _runners[id];
if (runner != null) {
runner.stop(_StopReason.pause);
} else {
_scheduled.remove(id)?.cancel();
task.status = UDownloadStatus.paused;
_changed(task);
}
}