Allow retrieving reference to AnimationTree objects

[ivxvm]

Lets say I have the following script:

@tool
extends AnimationTree

@export var is_attacking = true
@export var movement = 1.0

func _ready():
    pass
	
func _process(delta):
    super.set('parameters/StateMachine/Movement/blend_position', movement)

I would like to be able to get reference to Movement (AnimationNodeBlendSpace1d) object once and use it afterwards like this:

@tool
extends AnimationTree

@export var is_attacking = true
@export var movement = 1.0

var movement_ref

func _ready():
    movement_ref = super.get('parameters/StateMachine/Movement')
	
func _process(delta):
    movement_ref.blend_position = movement

This is useful both for convenience (when you use that field multiple times you don't have to put lengthy string path every time) and optimization (you don't have to perform any string lookups in this case, read/write by reference is O(1)), also to make it more consistent, because you already can do this for AnimationNodeBlendSpace2d, because its blend_position is object (with x and y), but can't do this for AnimationNodeBlendSpace1d because its blend_position is scalar.