rename extension from xaml to mxaml for maui xaml files, to workaroud the ava source gen issue

AvaloniaUI#7 (comment)

Author: juwens

Avalonia.Maui/Avalonia.Maui.csproj

@@ -23,8 +23,7 @@
 		<PackageReference Include="Avalonia.iOS" Version="$(AvaloniaVersion)" />
 	</ItemGroup>
     <ItemGroup Condition="$(TargetFramework.StartsWith('net7.0-windows'))">
-        <PackageReference Include="Avalonia.Desktop" Version="$(AvaloniaVersion)" />
-        <PackageReference Include="Avalonia.Win32" Version="$(AvaloniaVersion)" />
+         <PackageReference Include="Avalonia.Win32" Version="$(AvaloniaVersion)" /> 
     </ItemGroup>
     <ItemGroup>
 		<PackageReference Include="Avalonia" Version="$(AvaloniaVersion)" />

MauiSampleApp/AppShell.xaml renamed to MauiSampleApp/AppShell.mxaml

@@ -3,6 +3,6 @@
              xmlns:local="using:MauiSample"
              x:Class="MauiSample.AvaloniaApp">
     <Application.Styles>
-        <FluentTheme Mode="Light"/>
+        <!--<FluentTheme Mode="Light"/>-->
     </Application.Styles>
 </Application>