feat: Let the user provide a custom onScroll handler

[ythomop]

Feature Request

There are times when the scroll position is needed, e.g. to hide/show a floating tab bar.

Proposed Solution

This can be done exactly like the other scroll related handlers are overriden. The user provided onScroll can run after this library's onScroll

[e-younan]

Thanks for the enhancement request - I'll look into this tomorrow.

[e-younan]

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