[.NET 5] Start adding some project capabilities (#3)

Aligned with XA too, see dotnet/android#4383.

We'll start using Apple instead of iOS for these things at the IDE level since many
behaviors don't actually depend on iOS but also apply to tvOS, watchOS, and so on.

These capabilities go before other imports just in case additional packages/targets
from the SDK need to access them too.

msbuild/dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -3,4 +3,10 @@
   <Import Sdk="Microsoft.NET.Sdk" Project="Sdk.targets" />
     <!-- Default item includes (globs and implicit references) -->
   <Import Project="Xamarin.Shared.Sdk.DefaultItems.targets" />
+
+  <!-- Automatically supply project capabilities for IDE use -->
+  <ItemGroup>
+    <ProjectCapability Include="Apple" />
+    <ProjectCapability Include="Mobile" />
+  </ItemGroup>
 </Project>