Multibyte string extension does not described in composer.json schema.

[alexandrmazur96]

The most valuable methods in ISO3166 class rely on multibyte string comparisons: https://github.com/thephpleague/iso3166/blob/74a08ffe08d4e0dd8ab0aac8c34ea5a641d57669/src/ISO3166.php#L140-L153

So I think the library should describe ext-mbstring as a required extension in the composer.json schema.

[alcohol]

Fair enough.