docs: add side effect package.json in app structure (angular#37521)

With this change we add the special `package.json` which is used to mark the application free of non-local side-effects in the application source files section

PR Close angular#37521

Author: alan-agius4

aio/content/guide/file-structure.md

@@ -90,6 +90,7 @@ Angular components, templates, and styles go here.
 | `app/app.component.css`     | Defines the base CSS stylesheet for the root `AppComponent`. |
 | `app/app.component.spec.ts` | Defines a unit test for the root `AppComponent`. |
 | `app/app.module.ts`         | Defines the root module, named `AppModule`, that tells Angular how to assemble the application. Initially declares only the `AppComponent`. As you add more components to the app, they must be declared here. |
+| `app/package.json`              | This file is generated only in applications created using `--strict` mode. This file is not used by package managers. It is used to tell the tools and bundlers whether the code under this directory is free of non-local [side-effects](https://webpack.js.org/guides/tree-shaking/#mark-the-file-as-side-effect-free). |
 
 ### Application configuration files