Feature Request: Add thumbVisibility option for scrollbar control

[kihyun1998]

Problem description

Package: scrollable_positioned_list
Requesting a feature to add native support for scrollbar visibility control in ScrollablePositionedList. Currently, there's no direct way to control scrollbar visibility within the widget itself.

Steps to reproduce

1. Create a ScrollablePositionedList:

ScrollablePositionedList.builder(
  itemCount: items.length,
  itemBuilder: (context, index) => ListTile(
    title: Text('Item $index'),
  ),
)

Expected behavior

Should be able to control scrollbar visibility using a thumbVisibility property directly in ScrollablePositionedList, similar to ListView's behavior.

Actual behavior

Currently requires wrapping ScrollablePositionedList with a Scrollbar widget to control scrollbar visibility, leading to additional widget nesting.

Environment

• Flutter stable version
• scrollable_positioned_list: ^0.3.8

Additional details

This enhancement would improve API consistency with Flutter's core widgets and provide a better developer experience.