widgets/drag_handle library

1. double_sheet package
2. documentation
3. widgets/drag_handle.dart

double_sheet package

1. Libraries
2. controllers/sheet_gesture_controller
3. controllers/synchronized_scroll_controller
4. double_sheet
5. models/double_sheet_config
6. widgets/drag_handle
7. widgets/sheet_content
8. widgets/sheet_header
9. widgets/synchronized_double_sheet