Add next/reset function to AnimationStateMachine

[TokageItLab]

Closes godotengine/godot-proposals#2200
Closes godotengine/godot-proposals#5964

• Fixed glitch in the first frame of the transition. This was caused by the same animation being blended twice.
• Split start and travel requests. This allows for the generation of new paths in a single frame, as shown below:

if Input.is_action_just_pressed("ui_left"):
	asp.start("Start")
	asp.travel("End")

• Implements next(), which immediately transitions paths if the next path exist. This method allows transitions without changing the AnimationStateMachineTransition condition.

sti1.mp4

• Implement the reset option, which was added to NodeTransition under the name from_start (see also Add sync to NodeTransition and BlendSpace1D/2D and refactor sync in AnimationTree #62623), but since it is confusing with the StateMachine Start node, it is unified reset. I don't have any idea how this would be useful for non-looping animations, but for looping animations, it is useful if you want to resume the animation in the middle of a loop. Also it is necessary to avoid losing synchronization in the sync option.

sti2.mp4

state_machine_improvement_demo.zip

[SaracenOne]

This looks good, though as we discussed in messages, I'm considering feeling that the name should probably be chosen as opposed to the broad 'interrupt' function since it could have a variety of different meanings. I also still need to properly evaluate requirements for the 'reset' flag, but I'll do a full review soon!

[TokageItLab]

If reset is false, it would be correct to move to the further next state with processing no frame if the next state has already ended, but the current end process is a bit odd to begin with (I guess that it is probably related to #68986), so I will look at this in the next PR.

[reduz]

Looks good to me, but make sure to agree on the naming

[akien-mga]

Thanks!