Korge integrates tweens and easings, and it is fully integrated with coroutines for your coding pleasure.
Also animations files are first citiziens of the Korge Game Engine.
Korge supports SWF Adobe Flash/Animate files and can support other custom formats through extensions/plugins. For example it would be possible to create an exporter for After Effects or Apple Motion.
Korge defines a custom ANI file format for animations. Atlas based with sound support.
You can preview ANI/SWF files (as they will look in runtime) right in intelliJ, using Korge’s intelliJ plugin (that uses Korge itself for rendering!):