chore(node-bundle): mark private (#19139)

It was always supposed to be private. Oversight. 

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Author: iliapolo

tools/@aws-cdk/node-bundle/.projenrc.js

@@ -32,4 +32,6 @@ project.testTask.prependSpawn(project.compileTask);
 const buildAndTest = project.addTask('build+test');
 buildAndTest.spawn(project.testTask);
 
+project.addFields({ private: true });
+
 project.synth();