Make sure any constants defined at higher levels are not clobbered and define a couple more

Terminal.Gui/Terminal.Gui.csproj

@@ -16,6 +16,7 @@
     <RootNamespace>Terminal.Gui</RootNamespace>
     <AssemblyName>Terminal.Gui</AssemblyName>
     <SuppressNETCoreSdkPreviewMessage>true</SuppressNETCoreSdkPreviewMessage>
+    <DefineConstants>$(DefineConstants);JETBRAINS_ANNOTATIONS;CONTRACTS_FULL</DefineConstants>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
     <DebugType>portable</DebugType>