Add locale-aware sorting

[PIG208]

For mentions auto-completion, one of the comparators we have compares user names alphabetically using String.compareTo. Ideally, we should support locale-aware sorting using realm/user language settings.

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Quoting Greg from #1165 (comment) :

Yeah, I think using an ICU library via FFI is probably the full solution. That could be something we do ourselves once FFI support is more mature, as the Flutter roadmap calls for next year. Or it looks like people are working on building such a thing for everyone (and for the full scope of ICU functionality): package:icu4x, published by "unicode.org". Still a work in progress but has Unicode engineers working with the Dart team to build the things they need to make it all work:

• Ship binaries for Dart unicode-org/icu4x#4689

(And that is indeed using FFI; here's the Dart code implementing Collator.compare.)