Feature: Add extends mechanism

Problem/Motivation

In a previous prototype, it was possible to extends a design system with another. We still need this.

For example, https://system.design.orange.com/ is extending https://getbootstrap.com/

The related extends property, to eb added to design system definition (info.yml file), is already described in the definitions schema:

"extends": {
  "$ref": "#/$defs/uri",
  "description": "A design system URL Every components, styles, themes, variables, examples and libraries from this parent design system will be duplicated in the current design system, if their ID are not overriden."
},

Proposed feature

Process this property, retrieving the parent design system, and merging the parent definitions with the child definitions. If both design system have identical artefacts ID, only the children one is kept.

This is a recursive mechanism, if the parent also have a parent.

API changes

No, just addition. It is safe