Support twig-based rendering for SDCs

A first version is ready. I think we should still extend it to cover testing slots get rendered correctly also, i.e. a twig SDC containing some other twig SDC.

Then we should test that CE-rendered elements containing twig-SDCs works as expexted.
As outlined in the parent, the more complex case of twig SDCs containing CE-elements is something for another issue later.

added a test case for rendering a layout-SDC with slots using twig-based rendering, that works as expected also

so now only the more complex case of "CE-rendered elmeents in twig slots" is left, for which we have 📌 XB: Add support for rendering ce-elements in SDC slots Active . This can be tackled when all the rest came together.

So this MR introduces the helper setSdcCustomElementComponents() for deciding whether a SDC-component is rendered as twig in Drupal or via a custom-element in the decoupled frontend. So according to the plan 📌 First-class experience builder support Active this can be used by the XB-integration to enable CE-rendering for components where we have ce/nuxt-components registered. But this is not part of custom elements, but will have to become its own module or some optional XB-feature.

Given that the MR should be ready.

Passed technical review.

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