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)"`
