Auto merge of #26285 - mrobinson:animation-animation-state, r=jdm,emilio

Add ElementAnimationState and PossibleElementAnimationState

This refactor is preparation for implementing a specification
compliant transitions and animations processing model.

These data structures hold all the animation information about a single
node. Since adding, updating, and modifying animations for a single node
are all interdependent, it makes sense to start encapsulating animation
data and functionality into a single data structure. This also opens up
the possibility for easier concurrency in the future by more easily
allowing per-node mutexes.

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