These changes add support for
```
uv python pin 3.12 --global
```
This adds the specified version to a `.python-version` file in the
user-level config directory. uv will now use the user-level version as a
fallback if no version is found in the project directory or its
ancestors.
Closes#4972
This PR moves functions for finding user- and system-level config
directories to public functions in `uv_fs::config`. This will allow them
to be used in future work without duplicating code.
