Support for the `tensor` package

The tensor package in LaTeX is quite handy for writing tensors with mixed covariant and contravariant indices in an ergonomic fashion.

I think the package is quite small and would be fairly easy to add. I doubt I will get around to doing it myself, but there is a small change that it could happen. Would a PR for this package as a first-party extension be accepted should I ever get around to it?