Fix broken reference

Upsilon/BaseTypes/ScriptFunction.cs

@@ -1,6 +1,4 @@
 using System;
-using System.Collections;
-using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
 using System.Reflection;

Upsilon/Upsilon.csproj

@@ -10,4 +10,8 @@
       </Reference>
     </ItemGroup>
 
+    <ItemGroup>
+      <PackageReference Include="System.Collections.Immutable" Version="1.5.0" />
+    </ItemGroup>
+
 </Project>