Unexpected grapheme boundary with regional indicators (GB12)

[anba]

use icu::segmenter::GraphemeClusterSegmenter;

fn main() {
    let segmenter = GraphemeClusterSegmenter::new();
    let text = "🇺🇸🏴󠁧󠁢󠁥󠁮󠁧󠁿";

    segmenter
        .segment_str(text)
        .for_each(|i| println!("{}", i));
}

Reports the following break points:

0
4
8
36

which means "🇺🇸" is split into two graphemes, which should be disallowed per GB12.

CC: @aethanyc and @makotokato

---

Original bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1890007

[aethanyc]

This is fixed by #4536. Let me add the example as a test.