feat: Generate shell completions on build #232

Job	Run time
Extract crate metadata	5s
Ensure 'cargo fmt' has been run	6s
Minimum supported rust version	2m 12s
aarch64-unknown-linux-gnu (ubuntu-20.04)	1m 45s
arm-unknown-linux-gnueabihf (ubuntu-20.04)	1m 58s
arm-unknown-linux-musleabihf (ubuntu-20.04)	1m 46s
i686-pc-windows-msvc (windows-2019)	2m 45s
i686-unknown-linux-gnu (ubuntu-20.04)	1m 15s
i686-unknown-linux-musl (ubuntu-20.04)	1m 31s
x86_64-apple-darwin (macos-12)	2m 20s
x86_64-pc-windows-gnu (windows-2019)	4m 0s
x86_64-pc-windows-msvc (windows-2019)	3m 12s
x86_64-unknown-linux-gnu (ubuntu-20.04)	1m 25s
x86_64-unknown-linux-musl (ubuntu-20.04)	1m 26s
	25m 46s

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