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.

@grimreaper I see this is still marked as needs work, what is still pending on this one? Your patch looks like what I was thinking about implementing for this so I think you are on the right track here.

This is definitely a good start for what this module needs, there are some complex problems to solve left on this one. For the moment I added a workaround of adding this hook into my custom user theme:

/**
 * Implements template_preprocess_HOOK() for form_element.
 */
function hook_preprocess_form_element(&$variables) {
  $variables['description_toggle'] = TRUE;
  $variables['description_display'] = 'invisible';
}

The reason that the gin preprocess hook isn't triggered within Layout Builder is because the custom user theme is the active theme, not gin, so the workaround above makes sure that the necessary variables are set. I may also have some additional issues related to formatted text fields showing no description value to the preprocessor ... but yet somehow a description is still appearing?