USDZExporter: export object hierarchy

[mkeblx]

Description

Currently the object hierarchy is flattened on export. It would be useful for future improvements to not flatten.

From previous discussions this seems like something wanted, and broken down into a separate https://github.com/mrdoob/three.js/pull/26010#issuecomment-1538078064

Solution

I can submit a PR for this, before a number of additional USDZExporter improvements I'm working on (better material and texture handling, animation, etc).

Alternatives

You could leave as is.

Additional context

No response