add abstract method
Registers a disposer to run on deactivation (LIFO order).
The disposer may be sync (void) or async (Future<void>). Async
disposers are awaited inside runAll to preserve LIFO ordering.
If the feature has already been deactivated (e.g. the bag was
sealed while an async onStart was still awaiting), the disposer
is invoked immediately. Late-add of an async disposer is
fire-and-forget (the result is not awaited).
Implementation
void add(FutureOr<void> Function() disposer);