Move uvx shell completion to uvx --generate-shell-completion (#7511)

## Summary

Because a problem was found with Powershell and combining the generated
completion scripts for uv and uvx, let's try separating uv and uvx
command completion scripts.

The generated powershell script template can be seen in clap_complete
source, and it starts with `using` directives, which makes it impossible
(apparently) to concatenate two of those script outputs.

As a side effect, this is available under `uv tool run
--generate-shell-completion` too.

Fixes #7482

## Test Plan

- `eval "$(cargo run --bin uvx -- --generate-shell-completion bash)"`
- Test Powershell

crates/uv-cli/src/lib.rs

@@ -3247,6 +3247,9 @@ pub struct ToolRunArgs {
     /// By default, environment modifications are omitted, but enabled under `--verbose`.
     #[arg(long, env = "UV_SHOW_RESOLUTION", value_parser = clap::builder::BoolishValueParser::new(), hide = true)]
     pub show_resolution: bool,
+
+    #[arg(long, hide = true)]
+    pub generate_shell_completion: Option<clap_complete_command::Shell>,
 }
 
 #[derive(Args)]