Incorporating CLI functionality from Metanorma-related CLIs, such as Relaton, Plurimath, Pubid, Fontist...

Each of these CLIs provide necessary functionality.

When Metanorma is offered through the single binary, users are unable to access these CLIs.

However, Thor (which is what all of these gems use) allows for linking a "subcommand" to another CLI class.

So maybe we can have:
```sh
$ metanorma relaton fetch "iso 9001"
```

Thoughts?