feat: add --completions flag for shell completion generation #324

Job	Run time
clippy-checks	1m 23s
rustfmt-nightly-check	13s
build-and-test / build-artifacts-and-run-tests (true, true, false, false, x86_64-unknown-linux-gnu)	2m 32s
build-and-test / build-artifacts-and-run-tests (true, true, false, false, x86_64-unknown-linux-musl)	2m 50s
build-and-test / build-artifacts-and-run-tests (false, x86_64-unknown-linux-gnu)	1m 54s
build-and-test / build-artifacts-and-run-tests (true, true, false, false, armv7-unknown-linux-gnueabihf)	7m 35s
build-and-test / build-artifacts-and-run-tests (true, true, false, false, armv7-unknown-linux-musleabihf)	7m 29s
build-and-test / build-artifacts-and-run-tests (true, true, false, false, x86_64-pc-windows-msvc)	5m 58s
build-and-test / build-artifacts-and-run-tests (true, true, false, false, x86_64-apple-darwin)	5m 14s
build-and-test / build-artifacts-and-run-tests (true, x86_64-unknown-linux-gnu)	1m 53s
build-and-test / build-artifacts-and-run-tests (true, x86_64-unknown-linux-gnu)	2m 0s
build-and-test / build-artifacts-and-run-tests (true, true, false, false, x86_64-pc-windows-gnu)	6m 14s
build-and-test / build-artifacts-and-run-tests (true, true, false, false, aarch64-pc-windows-msvc)	1m 7s
build-and-test / build-artifacts-and-run-tests (true, true, false, false, aarch64-unknown-linux-musl)	6m 18s
build-and-test / build-artifacts-and-run-tests (true, true, false, false, aarch64-unknown-linux-gnu)	7m 24s
build-and-test / build-artifacts-and-run-tests (false, x86_64-unknown-linux-gnu)	1m 56s
	1h 2m 0s

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