Add client.css to astro build

Closes #3509357

Two parts to this MR

1. the addition of client.css to the Astro Hydration public directory that sets <astro-slot><astro-island> to be display: contents;. This is then included in the libraries.yml
2. Changes to the calculation of
   1. border outlines around components, regions, slots in the overlay - now they are based on child elements of the <astro-*>
   2. stack direction calculation (used to show either vertical or horizontal lines when dragging in grid/flex based slots) now finds the parent of the <astro-slot>

In 2.1 and 2.2, the code actually looks for display: contents; not the <astro-x> tagName which should be more robust.

Spotted an issue to address in a follow up

https://www.drupal.org/project/experience_builder/issues/3521034