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/lib.rs

@@ -132,7 +132,7 @@ mod tests {
     use indoc::{formatdoc, indoc};
     use temp_env::with_vars;
     use test_log::test;
-    use uv_configuration::Preview;
+    use uv_preview::Preview;
     use uv_static::EnvVars;
 
     use uv_cache::Cache;