Add a preview for spacing settings

We should show a preview for the spacing settings (xs, s, m, ...), maybe just as a legend beyond the space settings as a list of bars or something.

We need to extend the CSS for this by the theme, as these spacings are defined by the theme. So add a further library the theme is able to extend, like we did in drowl_layouts: YOURTHEME/drowl_layouts_layout_paragraphs_additions (https://www.drupal.org/project/drowl_layouts).

Also check out: https://www.drupal.org/project/drowl_paragraphs/issues/3273232