refactor: move hasListeners

Author: boschni

src/core/mutationObserver.ts

@@ -65,10 +65,6 @@ export class MutationObserver<
     }
   }
 
-  hasListeners(): boolean {
-    return this.listeners.length > 0
-  }
-
   onMutationUpdate(): void {
     this.updateResult()
     this.notify()

src/core/queryObserver.ts

@@ -95,10 +95,6 @@ export class QueryObserver<
     }
   }
 
-  hasListeners(): boolean {
-    return this.listeners.length > 0
-  }
-
   willFetchOnMount(): boolean {
     return (
       this.options.enabled !== false &&

src/core/subscribable.ts

@@ -20,6 +20,10 @@ export class Subscribable<TListener = Listener> {
     }
   }
 
+  hasListeners(): boolean {
+    return this.listeners.length > 0
+  }
+
   protected onSubscribe(): void {
     // Do nothing
   }