fix: possible memleak in CancelToken

[MSOB7YY]

hey thanks for the awesome package!

i noticed that in CancelToken implementation, we listen to a stream without keeping a reference to the subscription to later cancel

i haven't tested nor verified wether this makes an actual difference or not, but its better to keep things safe ig. i personally close all my subs when not needed

[Tienisto]

Sorry, after thinking about this change, the subscription should be automatically closed if the stream is done.
The stream is closed 2 lines above: _refController.close();.

Here is the documentation of StreamController.close:

  /// A stream controller will not complete the returned future until all
  /// listeners present when the done event is sent have stopped listening.
  /// A listener will stop listening if it is cancelled, or if it has handled
  /// the done event.
  /// A paused listener will not process the done even until it is resumed, so
  /// completion of the returned Future will be delayed until all paused
  /// listeners have been resumed or cancelled.

The listener is never paused, so it should close when the stream controller closes.
You are free to reopen this issue if I made a mistake.