Anim Blend Tree accessor override

[LeXXik]

A followup on #6211:

Anim Blend Tree overrides the inherited weight accessor from Anim Node:

engine/src/framework/anim/controller/anim-blend-tree.js

Lines 61 to 63 in b27ce45

	get weight() {
	this.calculateWeights();
	return this._parent ? this._parent.weight * this._weight : this._weight;

engine/src/framework/anim/controller/anim-node.js

Lines 62 to 68 in b27ce45

	set weight(value) {
	this._weight = value;
	}
	get weight() {
	return this._parent ? this._parent.weight * this._weight : this._weight;
	}

Solution:
The accessor should be removed from the Anim Blend Tree (or a setter added), and the Anim Node should add a conditional calculateWeights call:

get weight() {
    this.calculateWeights?.();
    return this._parent ? this._parent.weight * this._weight : this._weight;
}

Repro:
https://playcanvas.com/project/1205544/overview/blend-tree-forked