env: implement -S/--split-string

The GNU doc says:
```       
       -S, --split-string=S
              process and split S into separate arguments; used to pass multiple arguments on shebang lines
```
The code:
https://github.com/uutils/coreutils/blob/main/src/uu/env/src/env.rs

Tested in GNU with tests/env/env-S.pl          