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-static/src/env_vars.rs

@@ -225,6 +225,9 @@ impl EnvVars {
     /// Equivalent to the `--preview` argument. Enables preview mode.
     pub const UV_PREVIEW: &'static str = "UV_PREVIEW";
 
+    /// Equivalent to the `--preview-features` argument. Enables specific preview features.
+    pub const UV_PREVIEW_FEATURES: &'static str = "UV_PREVIEW_FEATURES";
+
     /// Equivalent to the `--token` argument for self update. A GitHub token for authentication.
     pub const UV_GITHUB_TOKEN: &'static str = "UV_GITHUB_TOKEN";