feat: add menu infra.

demo/Semi.Avalonia.Demo/Pages/MenuDemo.axaml

@@ -0,0 +1,20 @@
+<UserControl
+    x:Class="Semi.Avalonia.Demo.Pages.MenuDemo"
+    xmlns="https://github.com/avaloniaui"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    d:DesignHeight="450"
+    d:DesignWidth="800"
+    mc:Ignorable="d">
+    <StackPanel Margin="20" Spacing="20">
+        <Menu>
+            <MenuItem Header="File">
+                <MenuItem Header="Create" />
+                <MenuItem Header="Delete" />
+            </MenuItem>
+            <MenuItem Header="View" />
+            <MenuItem Header="Tools" />
+        </Menu>
+    </StackPanel>
+</UserControl>

demo/Semi.Avalonia.Demo/Pages/MenuDemo.axaml.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Semi.Avalonia.Demo.Pages;
+
+public partial class MenuDemo : UserControl
+{
+    public MenuDemo()
+    {
+        InitializeComponent();
+    }
+
+    private void InitializeComponent()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+}