Versions of extensions

I've rewritten an extension, so questions written for the old version will no longer work.
I also regularly have to use my local server to test new changes to an extension, so I don't inadvertently break other people's questions while debugging.

The editor should store multiple versions of extensions, and references to extensions should include the version. Declaring a new version should be an explicit step. There should be the option to always use the "development" version, but this should not be the default.

No files, tests, or entry points are named. Start by locating how the editor stores extensions and how questions reference them, then trace the workflow for declaring a new extension version. Done means older references remain usable, version declarations are explicit, and a non-default development version can be selected.

Written by the indexing model from the issue text.