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/anywhere_page.dart

@@ -0,0 +1,100 @@
+import 'package:flutter/cupertino.dart';
+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 AnywherePage extends StatelessWidget {
+  AnywherePage({
+    super.key,
+    required this.onChangedPlatform,
+  });
+
+  static const String route = 'anywhere';
+  static const String title = 'Context Menu Anywhere Example';
+  static const String subtitle = 'A context menu outside of a text field';
+
+  final PlatformCallback onChangedPlatform;
+
+  final TextEditingController _materialController = TextEditingController(
+    text: 'TextField shows the default menu still.',
+  );
+  final TextEditingController _cupertinoController = TextEditingController(
+    text: 'CupertinoTextField shows the default menu still.',
+  );
+  final TextEditingController _editableController = TextEditingController(
+    text: 'EditableText has no default menu, so it shows the custom one.',
+  );
+
+  static const String url = '$kCodeUrl/anywhere_page.dart';
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text(AnywherePage.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: ContextMenuRegion(
+        contextMenuBuilder: (context, primaryAnchor, [secondaryAnchor]) {
+          return AdaptiveTextSelectionToolbar.buttonItems(
+            anchors: TextSelectionToolbarAnchors(
+              primaryAnchor: primaryAnchor,
+              secondaryAnchor: secondaryAnchor as Offset?,
+            ),
+            buttonItems: <ContextMenuButtonItem>[
+              ContextMenuButtonItem(
+                onPressed: () {
+                  ContextMenuController.removeAny();
+                  Navigator.of(context).pop();
+                },
+                label: 'Back',
+              ),
+            ],
+          );
+        },
+        child: Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 64.0),
+          child: Column(
+            mainAxisAlignment: MainAxisAlignment.start,
+            children: <Widget>[
+              Container(height: 20.0),
+              const Text(
+                'Right click anywhere outside of a field to show a custom menu.',
+              ),
+              Container(height: 140.0),
+              CupertinoTextField(controller: _cupertinoController),
+              Container(height: 40.0),
+              TextField(controller: _materialController),
+              Container(height: 40.0),
+              Container(
+                color: Colors.white,
+                child: EditableText(
+                  controller: _editableController,
+                  focusNode: FocusNode(),
+                  style: Typography.material2021().black.displayMedium!,
+                  cursorColor: Colors.blue,
+                  backgroundCursorColor: Colors.white,
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}