[2.x] POC: Core design token API

Problem/Motivation

We are introducing a design token API in Core in #3531854: Add a Design Tokens & CSS variables API.

We need to challenge this API by using it in UI Skins.

Test how to provide UI for that.

Proposed resolution

Standard UI for the config entity:
- list page
- entity forms

Bulk creation for from a design token JSON file data.

Stop creating inline CSS but write into a file in public:// ?