Generate shell completion for uvx (#7388)

## Summary

Generate shell completion for uvx.

Create a `uvx` toplevel command just for completion by combining `uv
tool uvx` (hidden alias for `uv tool run`) with global arguments. This
explicit combination is needed otherwise global arguments are missing
(if they are missing, clap debug assertions fail when `uv tool run`
arguments refer to global arguments in directives like conflicts with).


Fixes #7258 

## Test Plan

- Tested using bash using `eval "$(cargo run --bin uv
generate-shell-completion bash)"`

crates/uv-cli/src/lib.rs

@@ -76,6 +76,13 @@ pub struct Cli {
     #[command(subcommand)]
     pub command: Box<Commands>,
 
+    #[command(flatten)]
+    pub top_level: TopLevelArgs,
+}
+
+#[derive(Parser)]
+#[command(disable_help_flag = true, disable_version_flag = true)]
+pub struct TopLevelArgs {
     #[command(flatten)]
     pub cache_args: Box<CacheArgs>,