Generated.xcconfig is only required at build time for iOS apps. In the
flutter create project template and example apps, Generated.xcconfig was
previously marked as a resource to be bundled into the built app.
This consolidates all of the non-template .gitignore rules into the top level .gitignore, to ignore common things more broadly, with less maintenance needed for the .gitignore files. Does not touch the templates, so that they still produce needed .gitignores as part of flutter create.
