[flutter_template_images] Support the new skeleton app template (#366)

This is the flutter_template_images part of https://github.com/flutter/flutter/pull/83530. It puts files that are used to build new Flutter apps into paths expected by the new mechanism (which supports more than one type of app template).

packages/flutter_template_images/CHANGELOG.md

@@ -1,3 +1,10 @@
+## 4.0.0
+
+* Move assets common to all app templates to a new `app_shared` directory
+  (relands changes reverted in 3.0.0).
+* Create `skeleton` directory and assets to support new app template
+  (formerly known as `list_detail_app`).
+
 ## 3.0.1
 
 * Fix maskable icon file names

packages/flutter_template_images/pubspec.yaml

@@ -2,7 +2,7 @@ name: flutter_template_images
 description: Image files for use in flutter_tools templates.
 repository: https://github.com/flutter/packages/tree/master/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: 3.0.1
+version: 4.0.0
 
 environment:
   sdk: ">=2.1.0 <3.0.0"