Create extension from alternative template

[rgov]

The default template uses React, but it is also possible to create an extension that does not use React, as https://github.com/foxglove/studio-extension-turtlesim as does.

It would be nice if I could either specify another template to use (for instance, a git repo URL) or otherwise just pass --without-react to use a specific non-React template.