testing/oils-for-unix: new aport

Add package oils-for-unix. It's the same project as oil, but the transpiled & compiled C++ version. Named according to https://github.com/oils-for-unix/oils/wiki/Oils-Packaging-Guidelines

I also added subpackages to provide /bin/sh and cmd:bash. I hope this is fine, but I can remove it if that isn't accepted for some reason.