Fix: Don't run the generation against classes

Author: gmiserez

src/Generator/EventSourceResolver.cs

@@ -33,7 +33,8 @@ public IEnumerable<EventSourceFile> FindEventSourceDefinitions()
                 EventSourceDefinitionVisitor visitor = new EventSourceDefinitionVisitor();
                 visitor.Visit(document.GetSyntaxRoot());
 
-                if (visitor.EventSource.HasEvents)
+                if (visitor.EventSource.IsInterface
+                    && visitor.EventSource.HasEvents)
                 {
                     yield return new EventSourceFile(document, visitor.EventSource);
                 }

src/Generator/Templates/EventSourceModel.cs

@@ -20,5 +20,6 @@ internal class EventSourceModel
 
         public bool HasEvents => Events.Any();
         public bool HasWriteMethods => WriteMethods.Any();
+		public bool IsInterface => !string.IsNullOrWhiteSpace(InterfaceName);
     }
 }