Add a `version` command

[RobjS]

So a user can run e.g. whippet version or whippet -v to get info on what version they're running.

[snim2]

How were you anticipating the version information would be stored?

[RobjS]

I hadn't really thought that far ahead! I guess maybe either in https://github.com/dxw/whippet/blob/main/src/Whippet.php, or in a file in a config/ folder? Probably as a protected property, or something along those lines.

[snim2]

Note the enhancement/version-command-to-return-whippet-version branch contains some work for this feature