feat: add separator component

Horizontal and vertical separator (divider) with CSS classes, ARIA
role="none", and data-orientation attribute. Includes dark mode support,
unit tests, and demos in all three dev targets.

bb.edn

@@ -39,6 +39,7 @@
               [ui.icon-test]
               [ui.sidebar-test]
               [ui.chip-test]
+              [ui.separator-test]
               [ui.theme-test])
    :task (let [{:keys [fail error]} (t/run-tests
                                       'ui.button-test
@@ -59,6 +60,7 @@
                                       'ui.icon-test
                                       'ui.sidebar-test
                                       'ui.chip-test
+                                      'ui.separator-test
                                       'ui.theme-test)]
            (when (pos? (+ fail error))
              (System/exit 1)))}