POC: Core design tokens API, UI Skins 2

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.

In addition of ui_suite_bootstrap.tokens.yml, should we provide in config/optional design token config entities to handle the scopes light/dark or default scope of CSS variables which is not :root???

I would say a big NO because:
- Bootstrap 5 has too many CSS variables, so importing hundreds of config entities is not very friendly
- currently in UI Skins 2, there is no check intended to compare config entity value against token default value
So this would produce a lot of inline CSS by default.