Add command accessors to `CommandGroupBuilder` #59

Job	Run time
Fmt check	10s
Test on macos with Rust stable (feat: default)	47s
Test on macos with Rust stable (feat: with-tokio)	52s
Test on macos with Rust 1.68.0 (feat: default)	2m 13s
Test on macos with Rust 1.68.0 (feat: with-tokio)	2m 29s
Test on ubuntu with Rust stable (feat: default)	23s
Test on ubuntu with Rust stable (feat: with-tokio)	22s
Test on ubuntu with Rust 1.68.0 (feat: default)	1m 4s
Test on ubuntu with Rust 1.68.0 (feat: with-tokio)	1m 10s
Test on windows with Rust stable (feat: default)	1m 8s
Test on windows with Rust stable (feat: with-tokio)	1m 24s
Test on windows with Rust 1.68.0 (feat: default)	1m 47s
Test on windows with Rust 1.68.0 (feat: with-tokio)	2m 1s
	15m 50s

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