Support component and astro_island render arrays

Motivation

XB produces render arrays which have type #component render elements for SDCs and type #astro_island for code components. By supporting those type of render arrays to be converted nicely into a tree of custom elements, we can enable the frontend to provide frontend-variants of SDCs and or just load the astro_island.

Proposed resolution

Improve handling of render-arrays such that they are not blindly wrapped into drupal-markup, but processed intelligently if possible.