Implements `runExit`

[arcanis]

Clipanion is currently a little annoying to use with single-command scripts because of its boilerplate. The new runExit API lets you write small CLIs even more easily:

import {Command, runExit} from 'clipanion';

runExit(class extends Command {
  async execute() {
    // ...
  }
});

[merceyz]

Helpers like this should probably not be attached to the Cli class so they can be tree shaken when not in use.

[arcanis]

But it's nice for readability 😢