Expose boolean operations on ShapePath

[KevinGutowski]

Right now you have to drop down to the private api to do this

let sketch = require('sketch')

let document = sketch.getSelectedDocument()
let selectedLayer = document.selectedLayers.layers[0]

console.log(selectedLayer.sketchObject.booleanOperation())

[matteogratton]

Up for this one.

More context: in order to create a boolean group, we have to perform more actions:

1. Create a Shape (which will become the group for the items)

   let shapeGroup = new Shape({
   frame: {
       x: layer.frame.x,
       y: layer.frame.y,
       width: layer.frame.width,
       height: layer.frame.height,
   },
   name: "Name",
   parent: someParent,
   layers: [],
   });

2. Add each layer to the shapeGroup (based on selection or other lists) and apply the boolean option we need:

   document.selectedLayers.layers.forEach((inner) => {
   inner.parent = shapeGroup;
   inner.sketchObject.setBooleanOperation(0);
   });

   where 0 = Union 1 = Substract 2 = Intersect 3 = Difference -1 = None