callback for page change

[BalduinLandolt]

To me, an extremely useful feature would be, if I could find out on which the pdf currently is.

I see two options for that:

1. the component forwards an event when the page changes, that indicates which page currently is selected.
2. the component could accept a callback function as a parameter, that gets executed when the page changes.

But maybe there are better ways... I'm not really good with Svelte yet.

Would that be a feature you might consider implementing?
Or would a PR implementing this be considered?