Motivation
See #93

At the moment, usethis show sonarqube-config requires [tool.usethis.sonarqube] and [tool.coverage.xml.output] to be specified, but there's no way to do that declaratively at the moment. A few thoughts here:

1. Maybe [tool.coverage.xml.output] can be standardized/assumed if not provided.
2. [tool.usethis.sonarqube] could be set using a usethis set interface or something like that; or possibly via a CLI arg to usethis tool sonarqube, which is mandatory if [tool.usethis.sonarqube] does not yet exist.
3. It's not clear what to do if the coverage tool is not being used - are we still going to assume that this [tool.coverage.xml.output] section needs to exist?
4. In general this show command feels a bit contingent on having already decided to use the sonarqube tool - should we call usethis tool sonarqube implicitly perhaps?
    
   Summary of desired enhancement
   Consider the above and decide on how to proceed