feat(flowchart): implement double circle node

The implementation uses two circles, inside each other.
A double circle node is opend with `()(` and closed with `)()`.

src/diagrams/flowchart/flowRenderer-v2.js

@@ -131,6 +131,9 @@ export const addVertices = function (vert, g, svgId) {
       case 'group':
         _shape = 'rect';
         break;
+      case 'doublecircle':
+        _shape = 'doublecircle';
+        break;
       default:
         _shape = 'rect';
     }