Move cli code to a hugr-cli subcrate

[aborgna-q]

Followup from my comment on the cli PR #1096 (comment).

The PR defines a new hugr cli interface with utilities. This is available directly on the hugr package, so it can be installed with cargo install hugr.
This requires the bin target to be defined directly in the main crate.

Currently the accessory code for the binary that handles arguments and tests is exposed directly under a cli feature in the hugr lib. We expect some internal projects to depend on this feature, but in general it's not a useful feature for users of the library.

We clean up this by following the structure of ripgrep:

• Define a hugr-cli subcrate with the library code and tests. Users can depend directly on this.
• Rename the hugr feature to _cli so it doesn't appear on docs.rs, and make it require hugr-cli.

This would help with modularising the code and reducing compile times.

[ss2165]

This builds nicely on to the crate split up work started by #1000

[aborgna-q]

Implemented in #1107