Fix ReactWindows-Desktop build break

`//vnect/Shared/Shared.vcxitems` is included by two project:

1. `//vnext/Microsoft.ReactNative/Microsoft.ReactNative.vcxproj`
2. `//vnext/Desktop/React.Windows.Desktop.vcxproj`

(2) is a static library that breaks due to compiler errors due to
`ReactNativeHost.cpp` including `XamlApplication.h` which in turn
includes `Xaml.XamlApplication.g.*` which requires
`XamlApplication.idl` to be generating them.  This won’t happen due to
the project type of (2).  Wrap `XamlApplication`-related code in a
macro and define it only for (1).

Author: sundaramramaswamy

vnext/Microsoft.ReactNative/Microsoft.ReactNative.vcxproj

@@ -124,6 +124,7 @@
         DISABLE_XAML_GENERATED_MAIN;
         REACTWINDOWS_BUILD;
         RN_PLATFORM=windows;
+        RNW_XAML_ISLAND;
         NOMINMAX;
         FOLLY_CFG_NO_COROUTINES;
         FOLLY_NO_CONFIG;

vnext/Microsoft.ReactNative/ReactNativeHost.cpp

@@ -13,7 +13,9 @@
 #include <winrt/Windows.Foundation.Collections.h>
 #include "IReactContext.h"
 #include "ReactInstanceSettings.h"
+#ifdef RNW_XAML_ISLAND
 #include "XamlApplication.h"
+#endif // RNW_XAML_ISLAND
 
 #include <Fabric/Composition/Modal/WindowsModalHostViewComponentView.h>
 #include <Fabric/WindowsComponentDescriptorRegistry.h>
@@ -103,9 +105,11 @@ IAsyncAction ReactNativeHost::ReloadInstance() noexcept {
     }
   }
 
+#ifdef RNW_XAML_ISLAND
   if (componentregistry->isXamlSupportRequired()) {
     winrt::Microsoft::ReactNative::Xaml::implementation::XamlApplication::EnsureCreated();
   }
+#endif // RNW_XAML_ISLAND
 
   ReactPropertyBag(m_instanceSettings.Properties()).Set(ReactNativeHostProperty(), get_weak());