Sam's created the template wizard gadget which works nicely. Check it out with adding this in your commonJS -
mw.loader.load( 'https://en.wikipedia.org/w/index.php?title=User:Samwilson/TemplateWizard.js&action=raw&ctype=text/javascript' );
Work is ongoing for the final bit T186672: Handle form submission, data type validation, and inserting template markup into the WikiText (Infobox wizard).
As long as most of the code is in JS, converting this into an extension should be straightforward. That'd allow us to write tests for it more easily, deploy everywhere at once and support localization more easily {{cn}}.