Remove Layout Builder dependency for the config and rendering of the Navigation sidebar

Problem/Motivation

In 📌 Remove Layout Builder dependency for the UI of Navigation admin config Active we remove the UI part of LB dependency, here we need to update the config format and rendering to remove the LB dependency.

This means that a new config format is needed, that is understandable by the moderns display builders being worked on, like XB and the PHP necessary to process this config into HTML will need to be brought in core.

Steps to reproduce

Save the navigation configuration, and check the config format being stored.

Proposed resolution

1. Define the new config format for modern display builders
2. Use it here
3. Bring in the supporting PHP necessary to render that config into HTML

Remaining tasks

User interface changes

none

Introduced terminology

probably a lot

API changes

probably quite a few things

Data model changes

Config format stored for the navigation config

Release notes snippet