fix(core): Stacks from 3rd-party libraries do not synthesize correctly

Even though this is arguably somewhat of an antipattern and user error:

- You should not be vending `Stack`s from libraries.
- All libraries should be using a single version of the CDK so that
  NPM can dedupe it (or use `peerDependencies` to avoid this altogether)

This is still a sharp edge that we can blunt a little by not using
the `instanceof` language feature.

No test because it needs a second copy of the CDK to expose it.

Fixes #10671.

packages/@aws-cdk/core/lib/private/synthesis.ts

@@ -153,7 +153,7 @@ function synthesizeTree(root: IConstruct, builder: cxapi.CloudAssemblyBuilder) {
       assembly: builder,
     };
 
-    if (construct instanceof Stack) {
+    if (Stack.isStack(construct)) {
       construct.synthesizer.synthesize(session);
     } else if (construct instanceof TreeMetadata) {
       construct._synthesizeTree(session);