Sample skinned assets without inverseBindMatrices

The glTF spec [says](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#skins) that `inverseBindMatrices` are optional.

Does this repo contain any sample assets that use skinning without the IBMs? We should probably have a test asset that checks whether this works in client implementations.

/cc https://github.com/KhronosGroup/glTF/issues/2411

Start with the glTF skin specification's `inverseBindMatrices` section and inspect the repository's existing sample assets for skinning examples. Use the linked Khronos issue for context, then add a sample asset that skins without `inverseBindMatrices`; done means the asset is suitable for client implementations to test this optional case.

Written by the indexing model from the issue text.