Our current text iterator is not aware of multi-code point graphemes.
Instead of simply incrementing an iterator one code point at a time, use
our Unicode grapheme segmenter to break text into fragments.
(cherry picked from commit f0105b473b9cc3446ec36359b559db9a8e78446d;
amended as usual for Unicode::Segmenter -> Locale::Segmenter, and to
tweak expected/multi-code-point-graphemes.txt for serenity's superior
emoji glyphs)
