join() should merge primitives within an instanced mesh

As discussed in https://github.com/donmccurdy/glTF-Transform/issues/1051, the `join()` operation bails out when it sees an instanced mesh (EXT_mesh_gpu_instancing), because it knows it won't be able to join that content to anything else in the scene. However, we *could* join primitives *within* an instanced mesh. The implementation of join() should be improved to do that where possible.

Start by locating the join() implementation and its existing handling for EXT_mesh_gpu_instancing. Review the related discussion in issue #1051, then determine how primitive merging can occur within an instanced mesh while preserving content that cannot be joined to other scene elements. Done means join() no longer bails out when those internal primitive joins are possible.

Written by the indexing model from the issue text.