Author tool migration to update existing Linux runners to add application shutdown handler #136783
Description
In #136780, @robert-ancell landed a tweak to the template that registers a no-op application shutdown handler so that Flutter developers are aware where they should add any code to be executed on application shutdown.
While it isn't strictly necessary to update older projects to have this code, it's a nice convenience. As such this is somewhat low priority.
The migration would apply the above-mentioned patch to the application's Linux runner, unless G_APPLICATION_CLASS(klass)->shutdown is already set in the project.
Parent issue: #136582