Add preview mode and use for warning in uv run (#3192)

Adds hidden `--preview` / `--no-preview` flags with `UV_PREVIEW`
environment variable support. Copies the `PreviewMode` type from Ruff.

Does a little bit of extra work to port `uv run` to the new settings
model.

Note we allow `uv run` invocations without preview and only use its
presence to toggle an experimental warning.

## Test plan

```
❯ cargo run -q -- run --no-workspace -- python --version
warning: `uv run` is experimental and may change without warning.
Python 3.12.2
❯ cargo run -q -- run --no-workspace --preview -- python --version
Python 3.12.2
❯ UV_PREVIEW=1 cargo run -q -- run --no-workspace -- python --version
Python 3.12.2
```

crates/uv-configuration/src/lib.rs

@@ -5,6 +5,7 @@ pub use constraints::*;
 pub use name_specifiers::*;
 pub use overrides::*;
 pub use package_options::*;
+pub use preview::*;
 pub use target_triple::*;
 
 mod authentication;
@@ -14,4 +15,5 @@ mod constraints;
 mod name_specifiers;
 mod overrides;
 mod package_options;
+mod preview;
 mod target_triple;

crates/uv-configuration/src/preview.rs

@@ -0,0 +1,37 @@
+use std::fmt::{Display, Formatter};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum PreviewMode {
+    #[default]
+    Disabled,
+    Enabled,
+}
+
+impl PreviewMode {
+    pub fn is_enabled(&self) -> bool {
+        matches!(self, Self::Enabled)
+    }
+
+    pub fn is_disabled(&self) -> bool {
+        matches!(self, Self::Disabled)
+    }
+}
+
+impl From<bool> for PreviewMode {
+    fn from(version: bool) -> Self {
+        if version {
+            PreviewMode::Enabled
+        } else {
+            PreviewMode::Disabled
+        }
+    }
+}
+
+impl Display for PreviewMode {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Disabled => write!(f, "disabled"),
+            Self::Enabled => write!(f, "enabled"),
+        }
+    }
+}