Fix 'jump to definition' for singleton operations (#646) (#681)

Resolves #646 

Tested locally, here's video proof: :)

![Screen Recording 2024-11-08 at 14 54
13-2](https://github.com/user-attachments/assets/e5ab9a1e-1976-4f72-ba63-672d5e2a74d6)

effekt/shared/src/main/scala/effekt/RecursiveDescent.scala

@@ -471,8 +471,7 @@ class RecursiveDescent(positions: Positions, tokens: Seq[Token], source: Source)
     nonterminal:
       `effect` ~> operation() match {
         case op @ Operation(id, tps, vps, bps, ret) =>
-          // TODO is the `true` flag used at all anymore???
-          InterfaceDef(IdDef(id.name), tps, List(Operation(id, Nil, vps, bps, ret) withPositionOf op))
+          InterfaceDef(IdDef(id.name) withPositionOf op, tps, List(Operation(id, Nil, vps, bps, ret) withPositionOf op))
       }
 
   def operation(): Operation =