Add method to return which character(s) require UCS-2 encoding

This method can be used to swap characters and keep the number of segments limited. 

A possible implementation of this method can be found here: https://github.com/Melusyn/text-message-utils/blob/0eda75e5cf1e9f0d29598a40950f0084fd6efaac/lib/models.js#L309