Refactor AnimationPlayer's API to be similar to Tween's API

[Diddykonga]

Inspiration:

The new implementation of Tween for 4.0 was quite nice, and am personally a fan of the Factory / Builder Pattern.
I think that AnimationPlayer would benefit from the same API style, and should still be able to provide for all of the same use cases as before.

Description:

AnimationPlayer's API could be refactored to work similar to Tween if we were to introduce a new Type such as: AnimationSequence (naming is just for the example)
Which could work similar to the Tweeners, implementation varying, in that you can use Builder Pattern on them to tweak their timing/playback/etc.

This could also allow, AnimationNode's more special animation features(AddxD, etc.), to be added as AnimationSequence methods.

Example GDScript:

var seq_main = AnimationPlayer.create_sequence("Main").auto_play(true)
seq_main.animation("Run").loops(-1).blend(0.0, 1.0) #Blend-In, Blend-Out
seq_main.additive_animation("Play_Guitar", 0.5).loops(-1).parallel() #Loop Count (-1 Infinite, 0 Once(Default))

var seq_attack_combo = AnimationPlayer.create_sequence("Tail_Attack_Combo")
seq_attack_combo.animation("Attack1").blend(0.25, 0.1)
seq_attack_combo.animation("Tail_Attack1").parallel() #Will run parallel with above animation, next animation below wont run until both are finished
seq_attack_combo.animation("Attack2").blend(0.0, 0.1)
seq_attack_combo.animation("Attack3").blend(0.0, 1.0)

var seq_ultimate_combo = AnimationPlayer.create_sequence("Ultimate_Combo")
seq_ultimate_combo.sequence("Tail_Attack_Combo")
seq_ultimate_combo.sequence("Claw_Attack_Combo")
seq_ultimate_combo.animation("Nature_Beam").blend(0.0, 3.0)

AnimationPlayer.play_sequence("Tail_Attack_Combo")
AnimationPlayer.queue_sequence("Ultimate_Combo")

AnimationPlayer.play_animation("Block", 0, 0.0, 0.5, false) #This should still work. It could have most common modifiers added as parameters
AnimationPlayer.queue_animation("Idle", -1, 1.0, 0.25, false) #Queue version

For simple uses, play/queue_animation("AnimationName", loops?, blend-in?, blend-out?, backwards?) should still be available. It will act as a quick helper method for building a single animation sequence, with the modifiers from the parameters applied.

As you may be able to tell from the example, this is not about changing what the AnimationPlayer plays, but instead changing how the user/developer interacts and uses the AnimationPlayer to play Animations. It is an enhancement, which still supports previous use cases, but will provide for much more as well.

Extra:

As this doesn't necessarily pertain to this topic, if this where to be accepted, we should look to separate/duplicate the AnimationNodeStateMachine out to just StateMachine : Node/Resource, as it seems a StateMachine would be useful outside of just Animation.