refactor(ui-shell): use $$restProps, remove cx

src/components/UIShell/GlobalHeader/HeaderNavItem.svelte

@@ -1,14 +1,15 @@
 <script>
   export let href = undefined;
   export let text = undefined;
-  import { cx } from '../../../lib';
 </script>
 
 <li>
   <a
-    class={cx('--header__menu-item')}
     role="menuitem"
     tabindex="0"
+    {href}
+    class:bx--header__menu-item={true}
+    {...$$restProps}
     on:click
     on:mouseover
     on:mouseenter
@@ -16,8 +17,7 @@
     on:keyup
     on:keydown
     on:focus
-    on:blur
-    {href}>
-    <span class={cx('--text-truncate--end')}>{text}</span>
+    on:blur>
+    <span class:bx--text-truncate--end={true}>{text}</span>
   </a>
 </li>