Add try_write_pattern_from_items

[sffc]

ok, if the goal is to make the monomorphization more explicit, I think I'd rather see a try_write_pattern_from_items that lives alongside try_write_pattern, and try_write_pattern can call into it.

Can be a follow-up.

Originally posted by @sffc in https://github.com/unicode-org/icu4x/pull/4873#discussion_r1592864564