Dart 3.9 / Flutter 3.35 [first LLM release] (#2714)

I got carried away with Gemini and basically rewrote CI and the release
process for the new LLM reality. This work was largely completed by
Gemini.

- Bump all SDK versions to the current beta (3.9.0-0)
- Run `flutter channel beta`
- Wrote `ci_script.dart` to replace the bash scripts
- Converted repository to pub workspace #2499 
- Added llm.md and release.md
- Added redirect for deprecated Samples Index

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

add_to_app/fullscreen/flutter_module_fullscreen/README.md

@@ -0,0 +1,14 @@
+# flutter_module
+
+An example Flutter module used in the Flutter add-to-app samples. For more
+information on how to use it, see the [README.md](../README.md) parent
+directory.
+
+## Getting Started
+
+For more information about Flutter, check out
+[flutter.dev](https://flutter.dev).
+
+For instructions on how to integrate Flutter modules into your existing
+applications, see Flutter's
+[add-to-app documentation](https://flutter.dev/docs/development/add-to-app).