Axis Registry: slant axis

https://github.com/google/fonts/blob/4bd3c05add902033e8d9e7f0c6258bf67e9a4814/axisregistry/slant.textproto has only a fallback for `0`, but @tphinney points out that a users of a family with a slnt would benefit from named style access to at least one slanted style. This will vary per family.