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-dev/src/compile.rs

@@ -4,7 +4,7 @@ use clap::Parser;
 use tracing::info;
 
 use uv_cache::{Cache, CacheArgs};
-use uv_configuration::{Concurrency, PreviewMode};
+use uv_configuration::{Concurrency, Preview};
 use uv_python::{EnvironmentPreference, PythonEnvironment, PythonRequest};
 
 #[derive(Parser)]
@@ -26,7 +26,7 @@ pub(crate) async fn compile(args: CompileArgs) -> anyhow::Result<()> {
             &PythonRequest::default(),
             EnvironmentPreference::OnlyVirtual,
             &cache,
-            PreviewMode::Disabled,
+            Preview::default(),
         )?
         .into_interpreter();
         interpreter.sys_executable().to_path_buf()