Allow for multiple MpCompatSyncWorkerAttribute on same method (#486)

There isn't anything preventing us from doing that normally, and we do that in some situations - especially for empty (and `shouldConstruct` only) sync workers.

Source/MpCompatAttributes.cs

@@ -495,7 +495,7 @@ public MpCompatSyncFieldAttribute(Type type, string instancePath, string fieldNa
             => this.instancePath = instancePath;
     }
 
-    [AttributeUsage(AttributeTargets.Method)]
+    [AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
     [MeansImplicitUse(ImplicitUseTargetFlags.WithMembers)]
     public class MpCompatSyncWorkerAttribute : Attribute
     {