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/context_menu_region.dart

@@ -0,0 +1,97 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+
+typedef ContextMenuBuilder = Widget Function(
+    BuildContext context, Offset offset);
+
+/// Shows and hides the context menu based on user gestures.
+///
+/// By default, shows the menu on right clicks and long presses.
+class ContextMenuRegion extends StatefulWidget {
+  /// Creates an instance of [ContextMenuRegion].
+  const ContextMenuRegion({
+    super.key,
+    required this.child,
+    required this.contextMenuBuilder,
+  });
+
+  /// Builds the context menu.
+  final ContextMenuBuilder contextMenuBuilder;
+
+  /// The child widget that will be listened to for gestures.
+  final Widget child;
+
+  @override
+  State<ContextMenuRegion> createState() => _ContextMenuRegionState();
+}
+
+class _ContextMenuRegionState extends State<ContextMenuRegion> {
+  Offset? _longPressOffset;
+
+  final ContextMenuController _contextMenuController = ContextMenuController();
+
+  static bool get _longPressEnabled {
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.android:
+      case TargetPlatform.iOS:
+        return true;
+      case TargetPlatform.macOS:
+      case TargetPlatform.fuchsia:
+      case TargetPlatform.linux:
+      case TargetPlatform.windows:
+        return false;
+    }
+  }
+
+  void _onSecondaryTapUp(TapUpDetails details) {
+    _show(details.globalPosition);
+  }
+
+  void _onTap() {
+    if (!_contextMenuController.isShown) {
+      return;
+    }
+    _hide();
+  }
+
+  void _onLongPressStart(LongPressStartDetails details) {
+    _longPressOffset = details.globalPosition;
+  }
+
+  void _onLongPress() {
+    assert(_longPressOffset != null);
+    _show(_longPressOffset!);
+    _longPressOffset = null;
+  }
+
+  void _show(Offset position) {
+    _contextMenuController.show(
+      context: context,
+      contextMenuBuilder: (context) {
+        return widget.contextMenuBuilder(context, position);
+      },
+    );
+  }
+
+  void _hide() {
+    _contextMenuController.remove();
+  }
+
+  @override
+  void dispose() {
+    _hide();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return GestureDetector(
+      behavior: HitTestBehavior.opaque,
+      onSecondaryTapUp: _onSecondaryTapUp,
+      onTap: _onTap,
+      onLongPress: _longPressEnabled ? _onLongPress : null,
+      onLongPressStart: _longPressEnabled ? _onLongPressStart : null,
+      child: widget.child,
+    );
+  }
+}