Refactor SdcController::preview() to use ComponentTreeHydrated, and update it to support nested components

Overview

#3446722: Introduce an example set of representative SDC components; transition from "component list" to "component tree" will add the remaining pieces for "tree" support, which #3455728: FieldType: Support storing component *trees* instead of *lists* and #3460856: Create validation constraint for ComponentTreeStructure laid the foundations for.

But \Drupal\experience_builder\Controller\SdcController::preview() already has:

    // @todo tree recursion — this only supports a flat list
    // @todo Refactor to use \Drupal\experience_builder\Plugin\DataType\ComponentTreeHydrated.

… because it currently uses custom logic to render these.

(// @todo support CSS + JS was handled in #3462450: Load assets inside preview <iframe> when components are added, we just forgot to delete that.)

Proposed resolution

• Replace the render array with a call to \Drupal\experience_builder\Plugin\DataType\ComponentTreeHydrated::toRenderable()
• … but that would lose the wrapping markup that allows the UI to identify each component. So make ::preview() recursively update the render array returned by ComponentTreeHydrated::toRenderable()
• 🎉 Thanks to #3460856: Create validation constraint for ComponentTreeStructure having landed, this can actually go ahead and update ComponentTreeHydrated to already explicitly supported components-in-slots 🚀

User interface changes

None. The whole point is that everything continues to work exactly the same, but:

1. with less special snowflake logic in SdcController::preview() — instead reusing the server-side foundations for rendering SDCs ⛄️🔥
2. a solid portion of what #3446722: Introduce an example set of representative SDC components; transition from "component list" to "component tree" would otherwise have to do to get previews of trees of components to render, is already taken care of 👍