Add sliding panel indicator.

[KilianShen]

Describe the bug Add sliding panel indicator.

To Reproduce Steps to reproduce the behavior: NONE

Expected behavior Add sliding panel indicator.

Screenshots If applicable, add screenshots to help explain your problem.

Additional context Add any other context about the problem here.

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Flutter Version [e.g. 1.12.13]
• sliding_up_panel Version [e.g. 1.0.0]

Sample main.dart Please provide a sample main.dart that reproduces this issue. The code provided here should be able to be run on its own without any external dependencies.

return Stack(
                      clipBehavior: Clip.none,
                      children: [
                        Container(
                          height: _ac.value *
                                  (widget.maxHeight - widget.minHeight) +
                              widget.minHeight,
                          margin: widget.margin,
                          padding: widget.padding,
                          decoration: widget.renderPanelSheet
                              ? BoxDecoration(
                                  border: widget.border,
                                  borderRadius: widget.borderRadius,
                                  boxShadow: widget.boxShadow,
                                  color: widget.color)
                              : null,
                          child: child,
                        ),
                        Positioned(
                          top: -25,
                          left: MediaQuery.of(context).size.width / 2-10,
                          child: Transform.rotate(
                            angle: _ac.value * 3.14,
                            child: const Icon(
                              Icons.keyboard_arrow_up,
                              color: Colors.white,
                            ),
                          ),
                        ),
                      ],
                    );