Allow currentPage (int) parameter instead of currentPageNotifier (ValueNotifier)

[ultimate-tester]

Greetings!

In my project, I rely on Bloc for State Management. I'd like to keep the current page index inside my bloc state, and can then rebuild my widgets if needed using Blocbuilder. Using a ValueNotifier to set the current page index introduces a bit of overhead, because I have the integer available in my state. I could simply set an integer for the currentPage and there's no need for a ValueNotifier anymore. This same principle applies to any state management library.

For this use case, I suggest to add a parameter called currentPage next to currentPageNotifier. Users can then choose to either input an integer, or input a ValueNotifier.

If you like this idea, I can write this up and create a PR in a short timespan. Let me know what you think.