support custom scope for sketch.find()
Current implemetion of `sketch.find()` only supports a single root for scope, like a page:

```js
sketch.find('Group', sketch.getSelectedDocument().pages[0])
```

But I'd like to find layers from user selection. So I have to use some kind of hack:

```ts
let selection: Layer[]
// not work, but I'd like to
// sketch.find('Group', selection)

//scope hack
function customFindRange(layers: Layer[], includingSelf: boolean): any {
    return {
        type: sketch.Types.Group,
        _isWrappedObject: true,
        sketchObject: {
            childrenIncludingSelf: function () {
                return layers.reduce((prev, layer) => {
                    prev.addObjectsFromArray(layer.sketchObject.childrenIncludingSelf(includingSelf))
                    return prev
                }, NSMutableArray.new())
            }
        }
    }
}
sketch.find('Group', customFindRange(selection, true));
```
Apparently, it's not stable.

Could you officially support `Layer[]` as scope?