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 `///`).

context_menus/test/utils.dart

@@ -40,6 +40,9 @@ List<TextSelectionPoint> globalize(
   RenderBox box,
 ) {
   return points.map<TextSelectionPoint>((point) {
-    return TextSelectionPoint(box.localToGlobal(point.point), point.direction);
+    return TextSelectionPoint(
+      box.localToGlobal(point.point),
+      point.direction,
+    );
   }).toList();
 }