Add support for implementing commands in the new CLI alone

[akalipetis]

Currently, we have only implemented project:init command in the new CLI. This command was separate and did not need interoperability with the Legacy CLI, in terms of authentication and configuration.

We should implement that foundation, in order to be able to work on new commands in the new CLI.

### Tasks
- [ ] https://github.com/platformsh/cli/issues/82
- [ ] https://github.com/platformsh/cli/issues/83

[dman-coders]

I'm guessing that the difference between legacy and only-new commands like this is also what causes the "did you mean" suggestion to be unaware of the new command(s) also?

https://platformsh.slack.com/archives/C05RZU4ES8G/p1723439099274829

upsun init

[CommandNotFoundException]      
Command "init" is not defined.  
Did you mean one of these?      
environment:init            
int:act                     

^ does not even suggest project:init, which is what I expected as the default.

Is there any way these commands can be indexed in a unified way, or is that a lot of work for small gain?

[akalipetis]

Yes, that's the reason this does not work, although init would not work even after this has been implements - you can use ify as a shortcut if you want.