windows absolute path handling not compatible with git bash

i have several problem on windows git bash (unfortunately, we need to life with it).

especially

- `import` does not respect the git bash syntax for full path, relative path works, but absolute path requires to have a path a that starts by `C:\` instead of `/c/`
- `source_directory` generates absolute path like `C:\` while i would expect `/c/` (my bash script does not like this in their command line)
- there is no `source_directory_native` that would be equivalent to `invocation_directory_native`

i am not expecting a fully automated solution (auto detecting the environment, on git bash use `/c/` and on cygwin use `C:\`, but i would like to have a more where only `/c/` is supported, and my justfile will only work on git bash or wsl.

thanks for this great tool !