Exposing the `PhoneNumber` class definition

Hi Nikolay,

Would you ever consider exporting the PhoneNumber class definition? When I used libphonenumber-js@1.9.6, I imported it from libphonenumber-js/build/PhoneNumber so that I could add a method to be inherited by all PhoneNumber instances. With the addition of the exports field in libphonenumber-js/package.json, this is no longer possible.

Going off of this example in the docs, I believe Google's libphonenumber exposes the class definition.

Thank you for your time.