When Swift Package Manager feature is enabled, create app and create plugin will have Swift Package Manager integration already added and will not need to undergo a migration.
Fixes https://github.com/flutter/flutter/issues/146371.
```
flutter config --enable-swift-package-manager
flutter create --ios-language swift --platforms ios,macos swift_app_name
flutter create --ios-language objc --platforms ios objc_app_name
flutter create --template=plugin --ios-language swift --platforms ios,macos swift_plugin_name
flutter create --template=plugin --ios-language objc --platforms ios objc_plugin_name
```
* Building shared C source code as part of the native build for platforms Android, iOS, Linux desktop, MacOS desktop, and Windows desktop.
* Sample code doing a synchronous FFI call.
* Sample code doing a long running synchronous FFI call on a helper isolate.
* Use of `package:ffigen` to generate the bindings.
