Add a uv cache size command

[yumeminami]

Summary

Implement uv cache size to output the cache directory size in raw bytes by default, with a --human option for human-readable output.

close #15821

[charliermarsh]

I'm not strongly opposed but it does seem like du -sh $(uv cache dir) might be equally sufficient here? In the linked issue, the idea is raised that we might track the cache size within the cache (to make reporting faster). That seems quite difficult, but if we aren't doing that, is there a good reason to support this over du -sh?

[yumeminami]

@charliermarsh

I think the main reasons is:

Cross-platform consistency: eliminating the need to call different command-line tools depending on the platform, du in Linux/macOS, but Windows doesn't have du(use Get-ChildItem in Windows). I think use the same code to handle is more elegant and maintanble than call different command-line tools