Anim Blend Tree accessor override

A followup on #6211:

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

https://github.com/playcanvas/engine/blob/b27ce4537fd417c40230b0ec1cc8b32f287c3e3b/src/framework/anim/controller/anim-blend-tree.js#L61-L63

https://github.com/playcanvas/engine/blob/b27ce4537fd417c40230b0ec1cc8b32f287c3e3b/src/framework/anim/controller/anim-node.js#L62-L68

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

Start with the linked definitions in src/framework/anim/controller/anim-blend-tree.js and src/framework/anim/controller/anim-node.js, then compare the inherited and overriding weight accessors. Verify the change against the linked blend-tree reproduction; done means the accessor behavior is consistent and the reported blend-tree issue no longer occurs.