feat: Ability for slide transitions to reverse when going back

[h3x4d3c1m4l]

Description

Hi! Thanks for an awesome library. When implementing FlutterDeckTransitionBuilder I noticed the transition always goes forwards. Even when navigating backwards instead of forward. Would you consider having support for backwards transitions? I think this will improve perceived flow for the people watching the presentation when the presenter goes backwards (e.g. to repeat onto something mentioned earlier).

Would you accept a PR for this?

[mkobuolys]

Hey, @h3x4d3c1m4l! Sorry for not responding to the issue... The feature is definitely not something on the priority list, but if you have a quick improvement laying around - sure, go ahead.