Auto merge of #134405 - rmehri01:x-completions, r=onur-ozkan

Generate shell completions for x as well

It would be nice to be have shell completions for both `./x` and `x` (installed with `cargo install --path src/tools/x`) instead of just `x.py`. This pr generates the corresponding completions for each shell in a similar way to `x.py` but under `x.<shell>` instead.

Author: bors