Split preview mode into separate feature flags (#14823)

I think this would give us better hygiene than a global flag. It makes
it easier for users to opt-in to overlapping features, such as Python
upgrades and Python bin installations and to disable warnings for
preview mode without opting in to a bunch of other features. In general,
I want to reduce the burden for putting something under preview.

The `--preview` and `--no-preview` flags are retained as global
overrides. A new `--preview-features` option is added which accepts
comma separated features or can be passed multiple times, e.g.,
`--preview-features add-bounds,pylock`. There's a `UV_PREVIEW_FEATURES`
environment variable for that option (I'm not sure if we should overload
`UV_PREVIEW`, but could be convinced).

crates/uv-dispatch/src/lib.rs

@@ -18,7 +18,7 @@ use uv_cache::Cache;
 use uv_client::RegistryClient;
 use uv_configuration::{
     BuildKind, BuildOptions, ConfigSettings, Constraints, IndexStrategy, PackageConfigSettings,
-    PreviewMode, Reinstall, SourceStrategy,
+    Preview, Reinstall, SourceStrategy,
 };
 use uv_configuration::{BuildOutput, Concurrency};
 use uv_distribution::DistributionDatabase;
@@ -99,7 +99,7 @@ pub struct BuildDispatch<'a> {
     sources: SourceStrategy,
     workspace_cache: WorkspaceCache,
     concurrency: Concurrency,
-    preview: PreviewMode,
+    preview: Preview,
 }
 
 impl<'a> BuildDispatch<'a> {
@@ -123,7 +123,7 @@ impl<'a> BuildDispatch<'a> {
         sources: SourceStrategy,
         workspace_cache: WorkspaceCache,
         concurrency: Concurrency,
-        preview: PreviewMode,
+        preview: Preview,
     ) -> Self {
         Self {
             client,