Auto merge of #111388 - clubby789:clap-complete, r=jyn514

Generate shell completions for bootstrap with Clap

Now that #110693 has been merged, we can look at generating shell completions for x.py with `clap_complete`. Leaving this as draft for now as I'm not sure of the best way to integration the completion generator. Additionally, the generated completions for zsh are completely broken (will need to be resolved upstream, it doesn't seem to handle subcommands + global arguments well).
I don't have Fish installed and would be interested to know how well completions work there.

Alternative to #107827

Author: bors