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_buttons_page.dart

@@ -0,0 +1,85 @@
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/material.dart';
+import 'package:url_launcher/url_launcher.dart';
+
+import 'constants.dart';
+import 'platform_selector.dart';
+
+class CustomButtonsPage extends StatelessWidget {
+  CustomButtonsPage({
+    super.key,
+    required this.onChangedPlatform,
+  });
+
+  static const String route = 'custom-buttons';
+  static const String title = 'Custom Buttons';
+  static const String subtitle =
+      'The usual buttons, but with a custom appearance.';
+
+  final PlatformCallback onChangedPlatform;
+
+  final TextEditingController _controller = TextEditingController(
+    text:
+        'Show the menu to see the usual default buttons, but with a custom appearance.',
+  );
+
+  static const String url = '$kCodeUrl/custom_buttons_page.dart';
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text(CustomButtonsPage.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 AdaptiveTextSelectionToolbar(
+                anchors: editableTextState.contextMenuAnchors,
+                // Build the default buttons, but make them look custom.
+                // Note that in a real project you may want to build
+                // different buttons depending on the platform.
+                children:
+                    editableTextState.contextMenuButtonItems.map((buttonItem) {
+                  return CupertinoButton(
+                    borderRadius: null,
+                    color: const Color(0xffaaaa00),
+                    disabledColor: const Color(0xffaaaaff),
+                    onPressed: buttonItem.onPressed,
+                    padding: const EdgeInsets.all(10.0),
+                    pressedOpacity: 0.7,
+                    child: SizedBox(
+                      width: 200.0,
+                      child: Text(
+                        CupertinoTextSelectionToolbarButton.getButtonLabel(
+                            context, buttonItem),
+                      ),
+                    ),
+                  );
+                }).toList(),
+              );
+            },
+          ),
+        ),
+      ),
+    );
+  }
+}