Support less common locale codes

[winrg]

I've started trying to write a Toki Pona localization of Chyrp Lite. Toki Pona's language code is tok and it has no associated country, so the locale regex doesn't know what to do with it.

This PR updates the regex and uses Intl's locale parsing, if available, to check locales for validity.

It works fine, at least when I'm developing on Linux, but untested on Windows.

[xenocrat]

Hello there,

Thank you for putting together this PR and making me aware of the limitation to localization. Rather than calling Locale::getDisplayName() twice conditionally, I would prefer to relax the regex I'm using in the directory iterator. I've committed Helpers: make locales() more permissive to do that.