Improve CustomElement::createFromRenderArray() to better handle entites that render to custom elements

Problem/Motivation

When a $render array for rendering an entity view mode is passed to createFromRenderArray(), and this renders into custom elements, we do want a proper custom-elements tree as result.

Steps to reproduce

see 📌 Views support Fixed

Proposed resolution

Take code from https://git.drupalcode.org/project/custom_elements/-/blob/8.x-2.x/src/Pr... and bring it into createFromRenderArray().
Also, we might have to do execute render() without the renderPlaceHolder() step, such that the first entity-builder renders into the render array with #custom_elements.