Port 'Collator' string comparison support to native

[ChrisLoer]

Port of mapbox/mapbox-gl-js#6270, see also design discussion at mapbox/mapbox-gl-js#4136.

Unfortunately this isn't a trivial port because we'll depend on platform-specific behavior. The rough plan is to implement this is to use built-in string comparison support on iOS and Android, and to bundle ICU (along with the necessary resource bundle) for other platforms. I suspect we won't figure out what we're actually going to do with Qt until the rubber hits the road.

/cc @anandthakker @nickidlugash @1ec5 @brunoabinader

[brunoabinader]

For Qt, we should probably follow the same procedure as adopted for other platforms using ICU as backend.

[tobrun]

This landed, follow up tickets #12268 and #12269 for Android/iOS