`xi-unicode` allows linebreaks in emoji sequences

The xi_unicode::LineBreakIterator allows breaks inside of a lot of the longer emoji sequences. For example:

assert_eq!(
    vec![(10, false), (14, false)],
    LineBreakIterator::new("️‍").collect::<Vec<_>>()
);

Not sure whether this is an actual bug in the code or whether the Unicode data is just outdated. In any case, since this library is used by quite a few projects, it might be worth investigating!