Move preview features into a dedicated crate (#15482)

## Summary

This is causing some cyclic dependencies issues for me, because these
can be used in virtually _any_ crate (like `uv-install-wheel`), which
then means that all of `uv-configuration` becomes a dependency, etc. I
think this should be a leaf crate so that we can safely depend on it
anywhere.

crates/uv-python/src/discovery.rs

@@ -8,7 +8,7 @@ use std::{env, io, iter};
 use std::{path::Path, path::PathBuf, str::FromStr};
 use thiserror::Error;
 use tracing::{debug, instrument, trace};
-use uv_configuration::Preview;
+use uv_preview::Preview;
 use which::{which, which_all};
 
 use uv_cache::Cache;