Replace jquery/once by core/once

Hello, a patch is added.

The fix looks mostly ok.

Some notes:

1. The core/once library should be added as dependency in widget_engine.libraries.yml file.
2. The once variable should be added as an argument to the respective JS files (like jQuery, Drupal, drupalSettings ...)

Thanks

Hi. I fixed this one with the requested changes at #4.

Automatically closed - issue fixed for 2 weeks with no activity.