Add pub add format guidance

[gspencergoog]

Description

This adds some short descriptions to the pub tools on how to add packages with other kinds of references, and in other sections. Gemini doesn't seem to know how to do this otherwise.

Also, converted the pub tool's argument to be an enum of the available tools, and added support for deps and outdated.

It's not an issue here because we're passing arguments directly, but I wish that pub add would parse "foo:{path:../foo}" (without the space after the colon, instead of "foo:{path: ../foo}" ), because it's a pain to have to escape the space or quote the arg.

[github-actions]

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