[unic-segment] Implement line breaking in Annex 14

[dead10ck]

Hello! I noticed unic-segment describes its purpose as implementing segmentation for graphemes, words, and sentences, as described in UAX#29. I'd like to propose a (possibly long-term) goal of adding line segmentation as described by UAX#14.

[clarfonthey]

Worth commenting that the unicode-linebreak crate has implemented this, for reference.