fix: enum CountryProperties is not defined at runtime in TypeScript

[alexdeleon]

Fixes #36 . Now you can write the following TypeScript code without runtime error:

import countries, { CountryProperty } from "country-codes-list";

function findCountry(code: string) {
 return countries.findOne(CountryProperty.countryCode, code);
}

[alexdeleon]

A better solution is: #41