Read from stdin if file is omited

[Entze]

Currently, clingofmt requires a path to be read from. Editors like Helix support custom formatters. However, the formatter (which would be clingofmt in this case) needs to support reading from stdin (see helix-editor/helix#9501). It is possible to workaround that limitation with shell-magic, however that is fragile.

Would you consider this feature? I can also try to help if needed.