feat: Add dragRegionBuilder to TabView for Borderless Window Dragging

[uiYzzi]

This PR introduces a new dragRegionBuilder property to the TabView component, allowing developers to easily implement window dragging functionality in borderless window applications.

Changes Made

Core Implementation:

• Added dragRegionBuilder property to TabView class in lib/src/controls/navigation/tab_view/tab_view.dart
• Modified build logic to wrap the entire tab strip with the drag region builder
• Maintained full backward compatibility with existing stripBuilder functionality
• Added comprehensive debug information for the new property

Documentation & Examples:

• Updated example app in example/lib/screens/navigation/tab_view.dart with practical usage demonstration
• Added dedicated showcase for borderless window dragging with window_manager integration
• Included clear usage patterns for developers

Testing:

• Added comprehensive test coverage in test/tab_view_test.dart
• Tests verify both dragRegionBuilder functionality and null handling
• All existing tests continue to pass

Related Issue

#1239

Usage Example

TabView(
  currentIndex: currentIndex,
  tabs: tabs,
  dragRegionBuilder: (context, child) {
    return GestureDetector(
      behavior: HitTestBehavior.translucent,
      onPanStart: (details) => windowManager.startDragging(),
      child: child,
    );
  },
)

Pre-launch Checklist

• I have updated CHANGELOG.md with my changes
• I have run "dart format ." on the project
• I have added/updated relevant documentation
• All tests pass successfully
• Example app demonstrates the new functionality