Fix XAML intellisense and hot reload in Visual Studio 2022 (#20851)

This fix is for MAUI devs that edit and debug sample projects in Visual
Studio (like `Maui.Controls.Sample.Sandbox`).


![image](https://github.com/dotnet/maui/assets/2523431/cfa16f54-056a-4dfd-89ee-ba3a4f0d9894)

Those sample projects normally don't set `UseMaui=true` since control
library projects are referenced directly in the solution. We need to
instead set `DefaultXamlRuntime=Maui` in order for Visual Studio
intellisense and XAML hot reload to work. VS uses either `UseMaui` or
`DefaultXamlRuntime` to determine the XAML runtime.

Author: spadapet

src/Compatibility/ControlGallery/src/Directory.Build.props

@@ -2,6 +2,7 @@
   <PropertyGroup>
     <SampleProject>true</SampleProject>
     <UseMaui Condition=" '$(UseWorkload)' == 'true' ">true</UseMaui>
+    <DefaultXamlRuntime Condition=" '$(UseWorkload)' != 'true' ">Maui</DefaultXamlRuntime>
   </PropertyGroup>
   <Import Project="../../../../Directory.Build.props" />
 </Project>

src/Controls/samples/Directory.Build.props

@@ -2,6 +2,7 @@
   <PropertyGroup>
     <SampleProject>true</SampleProject>
     <UseMaui Condition=" '$(UseWorkload)' == 'true' ">true</UseMaui>
+    <DefaultXamlRuntime Condition=" '$(UseWorkload)' != 'true' ">Maui</DefaultXamlRuntime>
     <WarningsNotAsErrors>$(WarningsNotAsErrors);XC0022;XC0023</WarningsNotAsErrors>
   </PropertyGroup>
   <Import Project="../../../Directory.Build.props" />

src/Essentials/samples/Directory.Build.props

@@ -2,6 +2,7 @@
   <PropertyGroup>
     <SampleProject>true</SampleProject>
     <UseMaui Condition=" '$(UseWorkload)' == 'true' ">true</UseMaui>
+    <DefaultXamlRuntime Condition=" '$(UseWorkload)' != 'true' ">Maui</DefaultXamlRuntime>
     <WarningsNotAsErrors>$(WarningsNotAsErrors);XC0022;XC0023</WarningsNotAsErrors>
   </PropertyGroup>
   <Import Project="../../../Directory.Build.props" />