Feature✨: Support promise step-handling out of the box

[josias-r]

The preventMove + continue API can already be used to easily write async step-handling.

This could be simplified by supporting promises out of the box.

So this:

onNext: async () => {
   boarding.preventMove();
   await myPromise();
   boarding.continue();
},

Could become this:

onNext: async () => {
   await myPromise();
},

Where continue & preventMove are auto handled by the library in the background in case a promise is returned from the handler.