Adds ai_recipe_generation sample (#2242)

Adding the demo app from my I/O talk. Because AI.

## 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 updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>

ai_recipe_generation/lib/util/extensions.dart

@@ -0,0 +1,13 @@
+import 'package:flutter/rendering.dart';
+
+extension SliverBreakpointUtils on SliverConstraints {
+  bool get isTablet => crossAxisExtent > 730 && crossAxisExtent < 1000;
+  bool get isDesktop => crossAxisExtent > 1000;
+  bool get isMobile => crossAxisExtent < 730;
+}
+
+extension BoxBreakpointUtils on BoxConstraints {
+  bool get isTablet => maxWidth > 730 && maxWidth < 1000;
+  bool get isDesktop => maxWidth > 1000;
+  bool get isMobile => maxWidth < 730;
+}