Add System.Collections.Immutable back to core

src/Discord.Net.Core/Discord.Net.Core.csproj

@@ -20,4 +20,7 @@
   <ItemGroup Condition=" '$(TargetFramework)' == 'net461' ">
     <PackageReference Include="System.ValueTuple" Version="4.4.0" />
   </ItemGroup>
+  <ItemGroup Condition=" '$(TargetFramework)' != 'netstandard2.1' ">
+    <PackageReference Include="System.Collections.Immutable" Version="5.0.0" />
+  </ItemGroup>
 </Project>