Add ext-intl requirement to composer.json

[urbanecm]

intl-extra makes use of the IntlDateFormatter class, which is only available in the intl extension of PHP. However, this dependency is not noted in composer.json, which means installation succeeds, but usage fails with a rather mysterious error:

PHP Fatal error:  Uncaught Error: Class "IntlDateFormatter" not found

To avoid this misunderstanding, add an explicit dependency on intl to composer.json, to make composer complain that the dependency is not met during installation.

[urbanecm]

I just realised I probably should've sent this to https://github.com/twigphp/Twig/issues/4702 instead. Done. Happy to make further changes if needed.

[stof]

PRs must indeed be sent o the main repo.