Add a command to print currently available script entry-points

[abhi-g80]

• [x] I have searched the issues of this repo and believe that this is not a duplicate.
• [x] I have searched the FAQ and general documentation and believe that my question is not already covered.

Feature Request

When working on projects with building command-line application, sometimes you would like to see the available script entry-points. I don't see any poetry command to do that. Something like this,

$ poetry show scripts

Available script entry-points

    twparser: Parse tweets into json
    validator: Validate schema of database

The description could be parsed by reading the function doc (if any).

[capatosta]

Bump, this'd be great