Add full XAML support for .NET MAUI compatibility

New features:
- MauiAppBuilderExtensions.UseOpenMauiLinux() for standard MAUI integration
- New template: openmaui-linux-xaml with full XAML support
- Standard MAUI XAML syntax (ContentPage, VerticalStackLayout, etc.)
- Resource dictionaries (Colors.xaml, Styles.xaml)
- Compiled XAML support via MauiXaml items

Templates now available:
- dotnet new openmaui-linux      (code-based UI)
- dotnet new openmaui-linux-xaml (XAML-based UI)

Usage:
    var builder = MauiApp.CreateBuilder();
    builder
        .UseMauiApp<App>()
        .UseOpenMauiLinux();  // Enable Linux with XAML

templates/openmaui-linux-xaml-app/MauiProgram.cs

@@ -0,0 +1,24 @@
+using Microsoft.Maui.Controls.Hosting;
+using Microsoft.Maui.Hosting;
+using OpenMaui.Platform.Linux.Hosting;
+
+namespace OpenMauiXamlApp;
+
+public static class MauiProgram
+{
+    public static MauiApp CreateMauiApp()
+    {
+        var builder = MauiApp.CreateBuilder();
+
+        builder
+            .UseMauiApp<App>()
+            .UseOpenMauiLinux()  // Enable Linux platform with full XAML support
+            .ConfigureFonts(fonts =>
+            {
+                fonts.AddFont("OpenSans-Regular.ttf", "OpenSansRegular");
+                fonts.AddFont("OpenSans-Semibold.ttf", "OpenSansSemibold");
+            });
+
+        return builder.Build();
+    }
+}