Add ShellCommand module for shell command stubs

[ZhongRuoyu]

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Follow-up to #17742, implementing @Bo98's idea #17742 (comment). A RuboCop has also been added to make sure each stub has its counterpart.

[MikeMcQuaid]

@ZhongRuoyu What's the thoughts on why include ShellCommand was a better fit here than class Repository < ShellCommand?

[Bo98]

Haven't checked but the command system might detect ShellCommand as a command itself if it's a subclass of AbstractCommand.

[MikeMcQuaid]

Nice work here, thanks @ZhongRuoyu!

I think a nice follow-up would be to rip out all the #: -handling logic for manpages now that we're not using them any more.