Move context menus out of experimental (#2134)

Now that the context menu code is on stable, move the sample out of
experimental.

Fixes https://github.com/flutter/samples/issues/2110

## 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>

context_menus/lib/image_page.dart

@@ -0,0 +1,83 @@
+import 'package:flutter/material.dart';
+import 'package:url_launcher/url_launcher.dart';
+
+import 'constants.dart';
+import 'context_menu_region.dart';
+import 'platform_selector.dart';
+
+class ImagePage extends StatelessWidget {
+  const ImagePage({
+    super.key,
+    required this.onChangedPlatform,
+  });
+
+  static const String route = 'image';
+  static const String title = 'ContextMenu on an Image';
+  static const String subtitle =
+      'A ContextMenu the displays on an Image widget';
+  static const String url = '$kCodeUrl/image_page.dart';
+
+  final PlatformCallback onChangedPlatform;
+
+  DialogRoute _showDialog(BuildContext context) {
+    return DialogRoute<void>(
+      context: context,
+      builder: (context) =>
+          const AlertDialog(title: Text('Image saved! (not really though)')),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text(ImagePage.title),
+        actions: <Widget>[
+          PlatformSelector(
+            onChangedPlatform: onChangedPlatform,
+          ),
+          IconButton(
+            icon: const Icon(Icons.code),
+            onPressed: () async {
+              if (!await launchUrl(Uri.parse(url))) {
+                throw 'Could not launch $url';
+              }
+            },
+          ),
+        ],
+      ),
+      body: Column(
+        mainAxisAlignment: MainAxisAlignment.center,
+        children: <Widget>[
+          ContextMenuRegion(
+            contextMenuBuilder: (context, offset) {
+              return AdaptiveTextSelectionToolbar.buttonItems(
+                anchors: TextSelectionToolbarAnchors(
+                  primaryAnchor: offset,
+                ),
+                buttonItems: <ContextMenuButtonItem>[
+                  ContextMenuButtonItem(
+                    onPressed: () {
+                      ContextMenuController.removeAny();
+                      Navigator.of(context).push(_showDialog(context));
+                    },
+                    label: 'Save',
+                  ),
+                ],
+              );
+            },
+            child: const SizedBox(
+              width: 200.0,
+              height: 200.0,
+              child: FlutterLogo(),
+            ),
+          ),
+          Container(height: 20.0),
+          const Text(
+            'Right click or long press on the image to see a special menu.',
+          ),
+        ],
+      ),
+    );
+  }
+}