feat: make VRS schema release info accessible in VRS-Python

[jsstevenson]

close #85

• use a hook during setup to grab the most recent release tag from the VRS submodule, then write it as an importable Python module (not necessarily the most elegant way to do this, but I think it's the only realistic option)
• This draft uses the scheme used by setuptools-scm](https://setuptools-scm.readthedocs.io/en/latest/usage/#default-versioning-scheme) to append commit hash/rev distance when the most recent tag isn't applied to the checked-out commit. I assume that the VRS submodule is tied to a branch and not a tag/commit due to its development status, and maybe this will be unnecessary once that stabilizes.
• ...not totally sure why this is failing in GH actions. It seems like git can return exit code 128 for a few reasons, and it's a little challenging to know why, but that'll obviously need to be resolved.
• very, very low priority. Was just something I was thinking about yesterday.