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/installation.rs

@@ -9,9 +9,9 @@ use tracing::{debug, info};
 
 use uv_cache::Cache;
 use uv_client::BaseClientBuilder;
-use uv_configuration::Preview;
 use uv_pep440::{Prerelease, Version};
 use uv_platform::{Arch, Libc, Os, Platform};
+use uv_preview::Preview;
 
 use crate::discovery::{
     EnvironmentPreference, PythonRequest, find_best_python_installation, find_python_installation,