move post-copier tasks to a smarter python script

[d-perl]

• If a plugin-managed widget has been removed, it will break the updater on the step where it attempts to run pyside6-uic on it. Failing this step should happen gracefully and report sensible errors to the user.
• In order to achieve this flexibility it would be better to run the post-update tasks in a python script to have more control over exit codes based on the errors encountered