Add Primitive Names

Currently a mesh has a name but the primitives that make it up do not.
This can make it difficult to identify important primitives for re-assigning materials etc.

Similar use case as for issue #1036

As is the case with "name" elsewhere, this would be informative for artists and not a reference within the glTF file itself.

At present I'm using an "extras" value for this in my implementation.

Review issue #1036 and the existing mesh and primitive naming behavior described in the issue. Determine how primitive names should support artist workflows without becoming references in the glTF file, and define the expected representation and acceptance criteria beyond the current extras-based approach.

Written by the indexing model from the issue text.