Add display-builder agnostic overview page for display modes
Closes #3564234
Problem / Motivation
There are several technical and architectural issues with the current "Manage display" implementation.
The first issue is the default view mode route dependency. Currently, the "Manage display" tab defaults directly to the default view mode edit form. To eventually deprecate or remove the default view mode in #2844203: Improve/Simplify situation around Default/Full view modes/view displays [4], we need a replacement entry point. While we could replace it with the "full" view mode, there's some technical complexity with how this would work with local tasks.
The second issue is display builder integration. Layout Builder, Display Builder, Canvas, and other display builders don't have a clear, standardized way to integrate with Field UI. The current architecture assumes Field UI is the primary display builder, making it difficult for alternative builders to provide their own management interfaces. We could accommodate multiple display builders working alongside each other much better than we do today.
This dependency also makes it difficult to eventually remove or replace Field UI's display building capabilities. By creating a neutral overview page that acts as a navigation hub, we can decouple the navigation from the specific display builder implementation.
Steps to reproduce
Navigate to Structure > Content types > [Content type] > Manage display. Observe that you are taken directly to the default display mode edit form. Notice that the route structure assumes Field UI is the only display builder.
Proposed resolution
Create a new overview page for "Manage display" that serves as a display-builder agnostic navigation hub. Add a new route at /admin/structure/types/manage/{bundle}/display that displays a table overview of all display modes for the bundle. This route is display-builder agnostic and serves as a neutral navigation hub.
Make the overview page the default local task, replacing the direct link to the default display mode. This provides a stable entry point that doesn't depend on any specific view mode.
