ENH: Ensure that the asset's `ParentGroup` can be specified on a pose-by-pose basis

Adds the ability to specify the assets def ParentGroup both for all poses as well as on a pose-by-pose basis, allowing one to neglect body sizes for specific poses.

A notable change here is also that null and undefined no longer have separate, distinct effects; both nullish values now imply inheritance from either the above-laying asset or group. The PoseType.DEFAULT singleton is now, instead, used for disabling inheritance (the latter being immune to accidental unforeseen consequences related to optional chaining).

Examples

// Inherit body sizes from the `BodyLower` group by default, but disable inheritance for the `AllFours` pose.
ParentGroup: {
    [PoseType.DEFAULT]: "BodyLower",
    AllFours: PoseType.DEFAULT,
}