util for Sketch

All the util nodejs API is available.

Additionally, 5 more methods are available:

- isNativeObject: return whether the argument is a native objc object
- getNativeClass: return the class name of a native object
- toArray: cast assimilated arrays (NSArray) to a proper JS array
- toObject: cast assimilated objects (NSDictionary) to a proper JS object
- toJSObject: cast any object or primitive into its proper JS counterpart. Note that NSDictionary and NSArray elements are not recursively converted unless an options object is passed as the second argument and options.recurse is set to true

_Because of CocoaScript, isNull is not going to work._