Add method to check if FastScroller is currently being displayed

[Diman96]

Hello, I would like to request the addition of a method in the FastScroller class that allows developers to determine if the fast scroll is currently being displayed. This feature would be helpful in various scenarios where the visibility status of the fast scroll might be needed to adapt the UI or handle specific user interactions.

Suggested implementation:
Please consider adding a method, such as isFastScrollVisible, to the FastScroller class that returns a boolean value indicating whether the fast scroll is currently being displayed or not.

This enhancement would provide valuable information about the fast scroll's visibility status and enable developers to make more informed decisions when designing their application's user experience.

Thank you for considering this feature request, and I appreciate your work on this library.

[Diman96]

Any updates ? @h6ah4i @zhanghai @drakeet