toJSON = (tween) => {
  const obj = {
    valuesStart: tween.valuesStart,
    valuesEnd: tween.valuesEnd,
  };
  return JSON.stringify(obj);
}
toJSON

Returns the {valuesStart, valuesEnd} objects
from a Tween instance in JSON string format.

@param {KUTE.Tween} tween the Tween instance
@returns {string} the JSON string