build(aio): check for obsolete plnkr.json and missing main files (#20165)

Also, remove `plnkr.json` for `service-worker-getting-started` guide,
since it is not used and ServiceWorker cannot work correctly in
plnkr/stackblitz anyway (e.g. no build step to re-compute hashes).
A zipper might be useful and can be added in a subsequent PR, but it is
currently broken (e.g. no dependency on `@angular/service-worker`).

PR Close #20165

aio/content/examples/lazy-loading-ngmodules/stackblitz.json

@@ -1,11 +1,10 @@
 {
   "description": "Lazy Loading Feature Modules",
-  "basePath": "src/",
   "files": [
     "!**/*.d.ts",
     "!**/*.js",
     "!**/*.[1,2].*"
   ],
-  "open": "app/app.component.ts",
+  "file": "src/app/app.component.ts",
   "tags": ["lazy loading"]
 }