chore(collapsible-section): add private custom css props to control header's border radius

Using a private custom CSS prop for controlling header's bottom border radius
can allow consumer components to conditionally style more details.
This will be particularly useful when implementing the drag to reorder feature.

Author: Kiarokh

src/components/collapsible-section/collapsible-section.scss

@@ -50,7 +50,9 @@
             rgb(var(--contrast-100))
         );
         border-radius: var(--border-radius-of-header)
-            var(--border-radius-of-header) 0 0;
+            var(--border-radius-of-header)
+            var(--limel-cs-open-header-bottom-border-radius, 0)
+            var(--limel-cs-open-header-bottom-border-radius, 0);
         &:hover,
         &:focus-visible {
             background-color: var(