Add completions sub-command to generate shell completion scripts #1389

Job	Run time
check-style	16s
check-generation	1m 18s
build-and-test (ubuntu-22.04, all)	4m 7s
clippy-lint	2m 59s
build-and-test (ubuntu-22.04, default)	4m 5s
build-and-test (windows-2022, all)	8m 40s
build-and-test (windows-2022, default)	8m 49s
build-and-test (macos-12, all)	4m 14s
build-and-test (macos-12, default)	4m 31s
	38m 59s

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