Provide an official way for tools to enumerate supported bundle versions

circup and possibly the vs code extension both encountered problems when we dropped the 6.x bundles. circup in particular hard coded the list of versions. if we provide an official way to enumerate supported bundle versions for the latest bundle, it will allow these consumers of the bundle to act appropriately when we add or remove a version.

Start by examining how supported bundle versions are currently represented and how circup hard-codes its list; check whether the VS Code extension has the same assumption. Done means consumers can enumerate the versions supported by the latest bundle and continue to behave correctly when versions are added or removed.

Written by the indexing model from the issue text.