LibCore: Add a constructor that sets the parent object

Author: https://github.com/kleinesfilmroellchen
Commit: dc318d3080
Pull-request: https://github.com/SerenityOS/serenity/pull/16715
Issue: https://github.com/SerenityOS/serenity/issues/14945
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/BenWiederhake

Userland/Libraries/LibCore/Promise.h

@@ -63,6 +63,10 @@ public:
 
 private:
     Promise() = default;
+    Promise(Object* parent)
+        : Object(parent)
+    {
+    }
 
     Optional<ErrorOr<Result>> m_pending_or_error;
 };