Proper PHP support, with PHPUnit tests & CI

This should help with easier access to data with less code in PHP (see #74).

Examples:

// Function imports
use function Annexare\Countries\continents;
$continents = continents();

// Fully-qualified, namespaced function calls
$countries = \Annexare\Countries\countries();

Credit for the effort 🚀 goes to @Radiergummi, thanks!

PHPUnit tests are automated with GitHub Actions and cover all exports. Tests run on the latest versions of PHP and PHPUnit.

NPM modules are up to date (well, except some Gulp dependencies which do not pass npm audit, but that's another story).
dist files are rebuilt with latest webpack & terser plugin.