[Feature] CLI autocomplete

Describe the user story

I'm a user and I'd like to type my commands faster by using my shell's auto-complete features.

Describe the solution you'd like

This is half a feature request for Clipanion, the CLI framework that we use (and that I wrote) to power Yarn's CLI.

I think the best would be to extend Clipanion in order to support a new function called omelette, which would generate a tree similar to the one that Omelette consumes:

omelette('hello').tree({
  how: {
    much: {
      is: {
        this: ['car'],
        that: ['house'],
      }
    }
    are: ['you'],
    many: ['cars', 'houses'],
  },
  where: {
    are: {
      you: ['from'],
      the: ['houses', 'cars'],
    },
    is: {
      // You can also add some logic with defining functions:
      your() {
        return ['house', 'car'];
      },
    }
  },
}).init()

Since Clipanion already operates with a kind of tree (which is then turned into a state machine), it should be fairly easy. The hardest part would likely be to find a way to power dynamic segments, such as package names. This is likely to be a followup task, though - having command names and options would already be a decent improvement.

Describe the drawbacks of your solution

It might prove more complex from an architectural standpoint than I expect.

Describe alternatives you've considered

We could let third-party write autocompletion scripts (which is what we've been doing for the duration of the v1), but now that we have a decent CLI framework that we control I think there's a lot we could do to improve the program UX. For example, typing yarn add typescript@<tab> and get offered the versions that yarn add -i would suggest.