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-cli/src/lib.rs

@@ -10,8 +10,8 @@ use clap::{Args, Parser, Subcommand};
 
 use uv_cache::CacheArgs;
 use uv_configuration::{
-    ExportFormat, IndexStrategy, KeyringProviderType, PackageNameSpecifier, PreviewFeatures,
-    ProjectBuildBackend, TargetTriple, TrustedHost, TrustedPublishing, VersionControlSystem,
+    ExportFormat, IndexStrategy, KeyringProviderType, PackageNameSpecifier, ProjectBuildBackend,
+    TargetTriple, TrustedHost, TrustedPublishing, VersionControlSystem,
 };
 use uv_distribution_types::{
     ConfigSettingEntry, ConfigSettingPackageEntry, Index, IndexUrl, Origin, PipExtraIndex,
@@ -19,6 +19,7 @@ use uv_distribution_types::{
 };
 use uv_normalize::{ExtraName, GroupName, PackageName, PipGroupName};
 use uv_pep508::{MarkerTree, Requirement};
+use uv_preview::PreviewFeatures;
 use uv_pypi_types::VerbatimParsedUrl;
 use uv_python::{PythonDownloads, PythonPreference, PythonVersion};
 use uv_redacted::DisplaySafeUrl;