Any configuration of transition animation should be decoupled from backstacks

[zach-klippenstein]

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I also think that any configuration of transition animation should be decoupled from backstacks, it's more fundamental than that. In non-compose, I want to tie it to WorkflowViewStub instead. I might have a notion of how to use ViewRegistry for that, if I can ever land the work to generalize that.

Originally posted by @rjrjr in https://github.com/square/workflow-kotlin/pull/304#r571125862

[rjrjr]

This can be added without breaking API, deferring.