Preserve fields

Problem/Motivation

When Layout Builder is enabled on an entity view display, Drupal removes all configurable fields from the render array because they are typically managed as field blocks within the Layout Builder UI. This behavior prevents fields from being available for inline editing with Edit Plus tools or being positioned independently in theme templates.

In some use cases, site builders need specific fields to remain in the render array (available in the content template variable) while still using Layout Builder for the rest of the page structure. This allows for hybrid page building approaches where:

1. Layout Builder manages structural sections and complex layouts
2. Certain fields remain available for template-based positioning and inline editing
3. Fields can be edited using Edit Plus's Change tool without entering Layout Builder

Currently, there's no way to preserve specific fields in the render array when Layout Builder is enabled.

Proposed resolution

Add a new "Preserved Fields" configuration section to the entity view display form that allows administrators to select which fields should remain in the render array when Layout Builder is enabled.