feat: Add autofocus for MenuItemButton (#139396)

MenuAnchor for Material 3 is great for contextual menus but there are some minor issues related to accessibility.
This PR aims to close the gap by adding `autofocus` to the menu item button so keyboard navigation is more intuitive. Otherwise, it becomes a mess to navigate through just keyboards.

Partially resolves #139395

## Additional Notes

I should mention, I have not written tests for this due to it's trivial nature. I also lack the experience of writing Flutter tests in general, so if someone feels inclined to take over this PR and add it they're welcome to.

packages/flutter/lib/src/material/menu_anchor.dart

@@ -856,6 +856,7 @@ class MenuItemButton extends StatefulWidget {
     this.requestFocusOnHover = true,
     this.onFocusChange,
     this.focusNode,
+    this.autofocus = false,
     this.shortcut,
     this.semanticsLabel,
     this.style,
@@ -897,6 +898,9 @@ class MenuItemButton extends StatefulWidget {
   /// {@macro flutter.widgets.Focus.focusNode}
   final FocusNode? focusNode;
 
+  /// {@macro flutter.widgets.Focus.autofocus}
+  final bool autofocus;
+
   /// The optional shortcut that selects this [MenuItemButton].
   ///
   /// {@macro flutter.material.MenuBar.shortcuts_note}
@@ -1140,6 +1144,7 @@ class _MenuItemButtonState extends State<MenuItemButton> {
       onFocusChange: widget.enabled ? widget.onFocusChange : null,
       focusNode: _focusNode,
       style: mergedStyle,
+      autofocus: widget.enabled && widget.autofocus,
       statesController: widget.statesController,
       clipBehavior: widget.clipBehavior,
       isSemanticButton: null,

packages/flutter/test/material/menu_anchor_test.dart

@@ -2229,6 +2229,45 @@ void main() {
       expect(find.text('leadingIcon'), findsOneWidget);
     });
 
+    testWidgets('autofocus is used when set and widget is enabled',
+        (WidgetTester tester) async {
+
+      listenForFocusChanges();
+
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Material(
+            child: Column(
+              children: <Widget>[
+                MenuAnchor(
+                  controller: controller,
+                  menuChildren: <Widget>[
+                    MenuItemButton(
+                      autofocus: true,
+                      // Required for clickability.
+                      onPressed: () {},
+                      child: Text(TestMenu.mainMenu0.label),
+                    ),
+                    MenuItemButton(
+                      onPressed: () {},
+                      child: Text(TestMenu.mainMenu1.label),
+                    ),
+                  ],
+                ),
+                const Expanded(child: Placeholder()),
+              ],
+            ),
+          ),
+        ),
+      );
+
+      controller.open();
+      await tester.pump();
+
+      expect(controller.isOpen, equals(true));
+      expect(focusedMenu, equals('MenuItemButton(Text("${TestMenu.mainMenu0.label}"))'));
+    });
+
     testWidgets('trailingIcon is used when set', (WidgetTester tester) async {
       await tester.pumpWidget(
         MaterialApp(