Re-create templates/app, add deprecation notices for app_shared a…

…nd `skeleton`. (#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.

packages/flutter_template_images/CHANGELOG.md

@@ -1,6 +1,7 @@
-## NEXT
+## 4.3.0
 
 * Updates minimum supported SDK version to Flutter 3.22/Dart 3.4.
+* Creates a copy of `app_shared` named `app` (`app_shared` will no longer be used).
 
 ## 4.2.1
 

packages/flutter_template_images/pubspec.yaml

@@ -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

packages/flutter_template_images/templates/app_shared/README.md

@@ -0,0 +1,10 @@
+**DO NOT UPDATE**: This directory is _deprecated_.
+
+The `skeleton` template is being removed, which as a result means that the
+`app_shared` directory is no longer needed (it was used to share template
+pieces between `app` and `skeleton`).
+
+If you are looking to update the `app` template, update
+[`templates/app`](../app/) directly.
+
+See also: <https://github.com/flutter/flutter/issues/160692>.

packages/flutter_template_images/templates/skeleton/README.md

@@ -0,0 +1,5 @@
+**DO NOT UPDATE**: This directory is _deprecated_.
+
+The `skeleton` template is being removed.
+
+See also: <https://github.com/flutter/flutter/issues/160692>.