`drupalorg skill:get` should return a list of all available skills when none is specified

[wimleers]

Just updated my drupalorg using drupalorg self:update.

Spotted the new skill:get. Tried it:

$ drupalorg skill:get

                                           
  Not enough arguments (missing: "name").  
                                           

skill:get [--full] [--] <name>

→ reaction: "Well okay 🫠 " 😂

Turns out that #341 created a table listing in the README listing the various skills. But that doesn't help one using drupalorg skill:get. Probably it's worth making drupalorg skill:get listing all available skills when none is specified?