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/custom_menu_page.dart

@@ -0,0 +1,96 @@
+import 'package:flutter/material.dart';
+import 'package:url_launcher/url_launcher.dart';
+
+import 'constants.dart';
+import 'platform_selector.dart';
+
+class CustomMenuPage extends StatelessWidget {
+  CustomMenuPage({
+    super.key,
+    required this.onChangedPlatform,
+  });
+
+  static const String route = 'custom-menu';
+  static const String title = 'Custom Menu';
+  static const String subtitle =
+      'A custom menu built from scratch, but using the default buttons.';
+
+  final PlatformCallback onChangedPlatform;
+
+  final TextEditingController _controller = TextEditingController(
+    text: 'Show the menu to see a custom menu with the default buttons.',
+  );
+
+  static const String url = '$kCodeUrl/custom_menu_page.dart';
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text(CustomMenuPage.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: Center(
+        child: SizedBox(
+          width: 300.0,
+          child: TextField(
+            controller: _controller,
+            maxLines: 4,
+            minLines: 2,
+            contextMenuBuilder: (context, editableTextState) {
+              return _MyContextMenu(
+                anchor: editableTextState.contextMenuAnchors.primaryAnchor,
+                children: AdaptiveTextSelectionToolbar.getAdaptiveButtons(
+                  context,
+                  editableTextState.contextMenuButtonItems,
+                ).toList(),
+              );
+            },
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+class _MyContextMenu extends StatelessWidget {
+  const _MyContextMenu({
+    required this.anchor,
+    required this.children,
+  });
+
+  final Offset anchor;
+  final List<Widget> children;
+
+  @override
+  Widget build(BuildContext context) {
+    return Stack(
+      children: <Widget>[
+        Positioned(
+          top: anchor.dy,
+          left: anchor.dx,
+          child: Container(
+            width: 200.0,
+            height: 200.0,
+            color: Colors.amberAccent,
+            child: Column(
+              children: children,
+            ),
+          ),
+        ),
+      ],
+    );
+  }
+}