[V2] Support Gin description toggle

Attaching patch for Composer usage.

POC state.

Another subject for discussion.

In https://git.drupalcode.org/project/gin/-/blob/8.x-3.x/js/description_tog..., the selectors are based on CSS classes not prefixed by "js-".

When making changes in gin_lb, I had to not use the glb_classes() Twig function to ensure CSS classes were preserved and the same as expected in Gin.

So I ended up looking at how glb_classes worked and step by step looking at how src/classes.json is generated. I miss something in postcss-glb-classes-generator/index.js to see how it finds the path and how exactly it extracts the CSS classes from Gin.

I think this is quite complicated, I understand the need of automation but:
- what if one day the classes.json file generation does not work anymore
- in terms of maintenance this can become heavy
- why not provide classes for gin_lb, as gin_lb provides its own CSS (or I missed an automatic inclusion of SCSS from Gin?)

Rebased patch.