chore: get rid of an instanceof (#19448)

We shouldn't be doing this, ever.


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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: rix0rrr

packages/@aws-cdk/core/lib/stack-synthesizers/_shared.ts

@@ -95,7 +95,7 @@ function collectStackMetadata(stack: Stack) {
   }
 
   function findParentStack(node: IConstruct): Stack | undefined {
-    if (node instanceof Stack && node.nestedStackParent === undefined) {
+    if (Stack.isStack(node) && node.nestedStackParent === undefined) {
       return node;
     }
 
@@ -183,4 +183,4 @@ export function stackTemplateFileAsset(stack: Stack, session: ISynthesisSession)
     packaging: FileAssetPackaging.FILE,
     sourceHash,
   };
-}
+}