Refactor AnimationStateMachine transition processes

[TokageItLab]

Remove the StateMachine grouping feature for now

• It is fundamentally completely the wrong architectural design and currently does not work properly at all. As noted in Separate Sub(Nested)StateMachine and StateMachineGroup processes godot-proposals#6130, GroupedStateMachine and NestedStateMachine need to be implemented differently. I will continue to work on the reimplementation following this PR.

Fading is now correctly cleared when teleportation occurs during fading

• Fixed Resetting the animation during the Xfade produced a strange condition #73113 (with Fix NodeOneShot doesn't respect fade-out when aborting and improvement #74190).

If there is nothing to play in the next State, that State is now skipped

• Previously, one frame of processing occurred even if there was nothing to play in the State (e.g. Immediate without Xfade, AtEnd without Xfade and Reset), but it is now skipped correctly. Fixed - AnimationTree: transition with xfade from a state machine start with wrong animation. #74752 (partially)

Add p_test_only option to blend_node() to fix a bug that caused glitching on the first frame

• The bug was caused by blending stacks. For example, when the same State was executed with blend amount 0 (for just retrieving remain time) and 1, the blend amount would be twice the value of one of the processes by overwriting wrong value to already existed blend amount on the blending stack, because the process that already existed on the blending stack could only have one amount value per reference (means AnimationNode), fixed. (This bug was mentioned somewhere, but I forget where)

[TokageItLab]

Superseded by #75759.