Vien

- The `create` and `recreate` commands without arguments will create a virtual
environment with the same interpreter that started `vien`, instead of the
interpreter available as `python3`
- Fixed: virtual environments were not cleared (`venv --clear`) before deletion
- Attempting to run the program on Windows will display a message stating that
the system is not fully supported

- Option `--project-dir` aka `-p` is now supported by all commands
- The `vien call -p` format (with the `-p` option after the command) is deprecated but still works

- `call` is now faster as it launches Python directly without spawning an extra
shell process

- A much shorter help message now appears when running `vien` without parameters
- Fixed: `call` printed debug message to stdout

- Trying to `call` a non-existent file now prints a short error message instead of an exception
- A message about a nonexistent environment displays a hint on how to create it

- Fixed: `$VIENDIR` did not change the storage directory of the virtual
environments