Add common LanguageIdentifier consts

[XAMPPRocky]

Currently if you want to have a language identifier that is valid at compile time you need to pull the proc macro langid!, which is a pretty heavy dependency in terms of compile times when your only use is langid!("en"). It would be nice if there's was a set of constants available for common language identifiers.

[zbraniecki]

We added the proc macro exactly to avoid defining limited number of consts. ICU does it and I quite frankly find it weird - it's against the whole philosophy of internationalization to say there's "a common language identifier".

Since I think it's more likely that you'll have a "common language identifier" in your project, I'd first try to figure out what it would take to make it easy for you to define your own const in your library/app. Is there something unic-locale can do to make that easier?

[XAMPPRocky]

I understand that motivation behind making the proc macro, and not wanting to have preferential treatment. However I think the current situation is quite annoying for my use case which is building higher level libraries and tooling on top of this library rather than a top level application.

When you use a proc macro in your tests and make a change in your crate all of those proc macros need to re-run every time, the only way around this is to move all the proc macro calls into a separate crate so that rust can compile it as a separate code unit.

Having a more limited selection is an acceptable trade off when I’m mainly concerned with testing that my library works with any language identifier.

[zbraniecki]

Having a more limited selection is an acceptable trade off when I’m mainly concerned with testing that my library works with any language identifier.

I understand that sentiment. I'm wondering if there's a way to make it easy for you to implement a const LanguageIdentifier rather than have unic-locale carry it.