fix(ui-shell): apply a11y navigation menubar attributes

Ref: https://github.com/carbon-design-system/carbon-components-svelte/pull/1073#issuecomment-1035061334

src/UIShell/GlobalHeader/HeaderNav.svelte

@@ -13,7 +13,7 @@
 </script>
 
 <nav {...props} class:bx--header__nav="{true}" {...$$restProps}>
-  <ul {...props} class:bx--header__menu-bar="{true}">
+  <ul {...props} role="menubar" class:bx--header__menu-bar="{true}">
     <slot />
   </ul>
 </nav>

src/UIShell/GlobalHeader/HeaderNavItem.svelte

@@ -26,7 +26,7 @@
   $: updateSelectedItems({ id, isSelected });
 </script>
 
-<li>
+<li role="none">
   <a
     bind:this="{ref}"
     role="menuitem"

src/UIShell/GlobalHeader/HeaderNavMenu.svelte

@@ -48,6 +48,7 @@
 />
 
 <li
+  role="none"
   class:bx--header__submenu="{true}"
   class:bx--header__submenu--current="{isCurrentSubmenu}"
   on:click="{(e) => {
@@ -65,6 +66,7 @@
 >
   <a
     bind:this="{ref}"
+    role="menuitem"
     tabindex="0"
     aria-haspopup="menu"
     aria-expanded="{expanded}"