Add preliminary support for menu-items

[justbur]

This is not extensively tested and any help along these lines would be appreciated.

[bdarcus]

Thanks much for this @justbur!

Just to confirm that with my initial tests using embark-act with bibtex-actions, it works; correctly applying the descriptions rather than the command names.

~I'm not sure if this is a which-key issue or (more likely) an embark issue, but I do note the default RET entry is showing the command name, rather than the description. I'll check with embark on that though.~

[minad]

@bdarcus The RET issue you mentioned is a embark issue.

[justbur]

I merged support for menu-items using a different approach in another branch