Move Border into its own file (#12364)

I'm about to add the BoxBorder and BorderDirectional classes to
this new file, but figured it would make review easier if the move
of the existing class happened first.

packages/flutter/lib/painting.dart

@@ -21,6 +21,7 @@ export 'src/painting/alignment.dart';
 export 'src/painting/basic_types.dart';
 export 'src/painting/border_radius.dart';
 export 'src/painting/borders.dart';
+export 'src/painting/box_border.dart';
 export 'src/painting/box_decoration.dart';
 export 'src/painting/box_fit.dart';
 export 'src/painting/box_shadow.dart';

packages/flutter/lib/src/painting/box_decoration.dart

@@ -10,7 +10,7 @@ import 'package:flutter/services.dart';
 
 import 'basic_types.dart';
 import 'border_radius.dart';
-import 'borders.dart';
+import 'box_border.dart';
 import 'box_shadow.dart';
 import 'decoration.dart';
 import 'edge_insets.dart';