When a user has downloaded an add-on as a zip file, it's not clear in
advance if this is a legacy add-on or a new extension. So they would
have to use trial and error, or inspect the zip file contents.
This uses a simple heuristic to check if the file is a legacy add-on,
and if so automatically calls the legacy operator instead.
The operator now show both extension and legacy add-on properties,
with the latter in a default collapsed subpanel.
Pull Request: https://projects.blender.org/blender/blender/pulls/121926
d568abea62