Make the item module public

[ElSamhaa]

Hello, thanks for forking and continuing the development of Skim.

I'm attempting to utilize Skim as a library without its UI. However, I encountered an issue when building the engine with the correct default criterion similar to what Skim uses in its binary. It seems that the item module is marked as private, preventing me from implementing something like this:

use skim::prelude::{item::{RankBuilder, RankCriteria}, *};

//...

  let search_factory = ExactOrFuzzyEngineFactory::builder()
    .rank_builder(Arc::new(RankBuilder::new(vec![
      RankCriteria::Score,
      RankCriteria::Begin,
      RankCriteria::End,
    ])))
    .build();

error[E0603]: module `item` is private

lotabout/skim#568
sayanarijit/xplr#678