Allow to add array custom properties

[andrewtelnov]

We have to add isArray attribute into property description to let Survey know that this property is an array and should deal with it as with array property. Here is the example:

Survey.Serializer.addProperty('survey', {
  name: 'customFunctions:calculatedvalues',
  className: 'calculatedvalue', isArray: true,
  displayName: "Custom Functions", category: "logic", visibleIndex: 0,
});