Build backend: Add template to uv init (#9661)

Add a preview option `uv init --build-backend uv --preview` that uses
the uv build backend when generating the project. The uv build backend
is in preview, so the option is also guarded by preview and hidden from
the help message and docs.

For https://github.com/astral-sh/uv/issues/3957#issuecomment-2518757563

crates/uv-configuration/src/project_build_backend.rs

@@ -4,6 +4,10 @@
 #[cfg_attr(feature = "clap", derive(clap::ValueEnum))]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub enum ProjectBuildBackend {
+    #[cfg_attr(feature = "clap", value(hide = true))]
+    #[cfg_attr(feature = "schemars", value(hide = true))]
+    /// Use uv as the project build backend.
+    Uv,
     #[default]
     #[serde(alias = "hatchling")]
     #[cfg_attr(feature = "clap", value(alias = "hatchling"))]