rules_uv for Windows

[classner]

This PR adds changes and functionality to make rules_uv work seamlessly on Windows. It does not make any additional assumptions about the environment (i.e., runs in regular windows batch or powershell, as well as msys2) - one limitation that I have not found a way around is the requirement for the create_venv targets to be named in a way that they are recognized as executable scripts by Windows (.cmd or .bat).

Let me know what you think!

[mark-thm]

@classner thanks for your work! In thinking about easing support for this, I'm tempted to turn rules_uv into a build action w/ a write_source_files target that's runnable to copy built files into the source tree. The gain of that would be getting rid of the bash (and new batch) scripts as we can have Bazel run the toolchain directly. It's been on my backburner to restructure things in this way, hoping to get to it in the next couple of weeks.