* Add automatic shell completion generation.
Add automatic generation of shell completion scripts for various shells
(the ones supported by `clap_complete`). The scripts can be generated
using the `generate-shell-completion` xtask, which outputs the shell
script to stdout.
* Improve shell completion generation xtask.
General improvements to both the shell completion generation as well as
the xtask package itself. Update the README to match the new additions.
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