Potentially expose sync feature of icu_provider

[aapoalas]

At least in Nova's usage, dropping of GC'd heap items (and generally will) can happen on threads other than the main JavaScript runtime thread.

For ZonedDateTime specifically, it is not currently Send due to using an Rc<Box<[u8]>> inside of icu_provider crate. The crate does offer a sync feature which would (presumably) switch to using Arc.

I assume this would be necessary for us to use temporal_rs safely; but maybe I'm mistaken?

[LeandroVandari]

imo it at least should be documented that ZonedDateTime requires icu_provider's sync feature in order to become Send + Sync. While building a multi-threaded program with it, i ran into an issue with the lack of implementation of those traits, and had to dig into icu_provider's source code to figure out the issue (and find out that the sync feature existed)