Introduce a Lupus Client Side Rendering theme

Motivation

- Make it as easy as possible to start with Lupus Decoupled.
- The CSR approach has the advantage that it inherits the backend authention, since in runs in the backend/Drupal context. This makes it a good fit for rendering previews, thus solving 🐛 Previews fails if user is not logged into the frontend Active .

Proposed resolution

JS: Have the JS-framework generate the client-side app and dump in a suiting dir, e.g. web/_nuxt for nuxt.
Theme: Completely replace the page template with a loader / copy of the HTML shell (index.html) generated by the JS

This will trigger the client-side JS app to be loaded instead of the Drupal site. With nuxt, this should all work out of the box and the app will load. However, it will fetch the ce-api content via a new API request. As a follow-up we can improve this:

Phase2: Ideally we ship the ce-api response already with the Drupal request that generated the JS-app-shell and provide it in a pre-defined way, so some framework glue-code can pick it up.