src/locales/: Add new file 'countries.php', which at present just provides a function for obtaining a "map" (or "associative array", in PHP terms), mapping all country-codes to their respective country-names, as well as a function to check if a given string represents a valid country code.
src/address.php: Re-implement 'getCountriesMapForSelectField' using new MyPHPLibs\Locales\countriesMap function.