Use Promise for router onBefore

[blank-software]

Currently, the official documentation here states:

Returning false from the onBefore() callback will cause the visit to be cancelled.

And it provides an example using the built-in confirm() function. That sounds all good, but does anyone actually use the built-in confirm() function? I think the more common use case would be to show a confirmation message with something like SweetAlert, which currently is not possible because onBefore() does not support async callbacks. The built-in confirm() is a rudimentary function that blocks the thread, but if you want to implement a proper confirmation dialog, onBefore() is actually useless for that.

I would suggest that the router should accept a promise for onBefore, and it should perform the visit only when the promise is fulfilled successfully.