Consider removing usages of Disposable in favor of `disposable` method

[evanweible-wf]

From https://github.com/Workiva/state_machine/issues/61#issuecomment-812258442

Since this is a fairly low-level package, it would be nice to remove any dependencies that aren't strictly necessary. Disposable is one of those, since Machine could provide that functionality via a dispose() method that is easily integrated into an API like Disposable if desired by the consumer.

[haodong404]

Any updates? The intl's version used by w_common is too old(intl < 1.19.0) for flutter >= 3.22.