Re-create templates/app, add deprecation notices for app_shared and skeleton. (flutter#8336)

Part of flutter/flutter#160673, which removes `--template skeleton`.

This PR copies the `templates/app_shared` directory, which was only used to share code between `templates/app` and `templates/skeleton`, in preparation for removing `skeleton` (and making the existence of `app_shared` obsolete).

Issue tracking final cleanup: flutter/flutter#160692.

Author: matanlurey

packages/flutter_template_images/CHANGELOG.md

@@ -2,7 +2,7 @@ name: flutter_template_images
 description: Image files for use in flutter_tools templates without adding binary files to flutter/flutter.
 repository: https://github.com/flutter/packages/tree/main/packages/flutter_template_images
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+flutter_template_images%22
-version: 4.2.1
+version: 4.3.0
 
 environment:
   sdk: ^3.4.0