Add support for optional --description in uv init (#10209)

## Summary
Closes #7913 by adding an optional `--description` argument to `uv init`
that fills the description field in the pyproject.toml with the supplied
arg value.

Updated `uv init` docs to describe this new optional argument.
<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan
Added snapshot tests in `uv/crates/uv/tests/it/init.rs` to test this
functionality.
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---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/uv-cli/src/lib.rs

@@ -2568,9 +2568,13 @@ pub struct InitArgs {
     ///
     /// By default, adds a requirement on the system Python version; use `--python` to specify an
     /// alternative Python version requirement.
-    #[arg(long, conflicts_with_all=["app", "lib", "package", "build_backend"])]
+    #[arg(long, conflicts_with_all=["app", "lib", "package", "build_backend", "description"])]
     pub r#script: bool,
 
+    /// Set the project description.
+    #[arg(long, conflicts_with = "script")]
+    pub description: Option<String>,
+
     /// Initialize a version control system for the project.
     ///
     /// By default, uv will initialize a Git repository (`git`). Use `--vcs none` to explicitly