Extend OutlinedBorder for use in Chip with flutter 2.0

[wietsebuseyne]

In flutter 2.0, a Chip requires a OutlinedBorder. For this plugin to keep working with the Chip (and potentially other) classes, the PolygonBorderclass should extend OutlinedBorder.

I implemented and tested the change. I will create a pull-request later tomorrow.

The necessary changes are:

1. extend OutlinedBorder instead of ShapeBorder
2. use the OutlinedBorder side variable instead of the border variable that is used right now
3. implement copyWith:

   @override
   OutlinedBorder copyWith({BorderSide side}) {
   if (side == null) return this;
   return PolygonBorder(
     sides: sides,
     side: side,
     rotate: rotate,
     borderRadius: borderRadius,
   );
   }

[wietsebuseyne]

Fixed in https://github.com/leonardocaldas/flutter-polygon-clipper/pull/10