Remove --template=skeleton and add a placeholder error message instead. (#160695)

Closes https://github.com/flutter/flutter/issues/160673.

Does the following:

- Renames `FlutterProjectType` to `FlutterTemplateType`; did some
enhanced enum cleanups while at it
- Creates a hierarchy of `RemovedFlutterTemplateType` from
`ParsedFlutterTemplateType`
- Removes the `skeleton` directory
- Merges `app_shared` back into `app` (no longer required now that
`skeleton` is removed)

Final cleanups are tracked in
https://github.com/flutter/flutter/issues/160692.

(Added @zanderso just to spot check this is what he meant by
https://github.com/flutter/flutter/issues/160673#issuecomment-2557742347)

dev/bots/analyze.dart

@@ -2443,7 +2443,7 @@ final String _kTemplateRelativePath = path.join(
   'packages',
   'flutter_tools',
   'templates',
-  'app_shared',
+  'app',
   'windows.tmpl',
   'runner',
 );