nodeToSketchGroup - flatten option

[kdzwinel]

As discussed in chrisvxd/story2sketch#20

Before

nodeToSketchGroup(node);

(group A)
  (group B)
     (group C)
        layer

After

nodeToSketchGroup(node);

(group C)
   layer

❓Or should it be group A? Or maybe some new group that contains names of all A, B & C? 🤔

[yakunins]

An attempt to make ungrouping with data-sketch-ungroup attribute:

1. in nodeTreeToSketchGroup.js see line 24 and line 41
2. ungroup detection function in /helpers/symbolAttributes/utils.js see line 9