Provide a WordSeparator which uses rust_icu

[mgeisler]

With #332 merged, it is now possible to customize the way text is split into words via the WordSeparator trait. From the discussion in #220, I understand that the rust_icu crate is the gold standard for breaking text into words. We should add a WordSeparator implementation which uses that library.

[mgeisler]

Hi @tavianator and @sirwindfield, you were active in #220 where we talked about rust_icu. Would any of you be interested in adding support for it?

[mainrs]

I don't have time right now (sadly) due to university to contribute to open source in general :( I'd be happy to review a PR though. It shouldn't take that much time compared to implementing it :)

[tavianator]

I can give it a shot soon

[mgeisler]

@sirwindfield I understand completely! I only asked in case you and @tavianator hadn't seen the issue yet :-)

@tavianator, that would be awesome, thanks!

There's no stress — I think I'll make a new release in the next 1-2 weeks, but we can make another any time after that with rust_icu support (the releases are almost completely automated, so it's easy to make new ones).

My goal for the next release is to make the wrap algorithm pluggable via a trait (#325). I looked at it a little today and I think I'll be able to do that this week.

[mgeisler]

After #438, this would either be another variant in the WordSeparator enum, or perhaps it could use the new WordSeparator::Custom variant directly.