Add support for --dry-run mode in uv lock (#7783)

This PR adds support for `uv lock --dry-run`, as described in issue
#6408.

One thing to note: this functionality, as implemented, isn't limited to
`-U` (if someone adds a dependency to the project's `pyproject.toml`,
the plan will include these changes).

---------

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

crates/uv-cli/src/lib.rs

@@ -2903,6 +2903,13 @@ pub struct LockArgs {
     #[arg(long, env = EnvVars::UV_FROZEN, value_parser = clap::builder::BoolishValueParser::new(), conflicts_with = "locked")]
     pub frozen: bool,
 
+    /// Perform a dry run, without writing the lockfile.
+    ///
+    /// In dry-run mode, uv will resolve the project's dependencies and report on the resulting
+    /// changes, but will not write the lockfile to disk.
+    #[arg(long, conflicts_with = "frozen", conflicts_with = "locked")]
+    pub dry_run: bool,
+
     #[command(flatten)]
     pub resolver: ResolverArgs,