feat(NavigationMenu): handle children on vertical orientation (#2384)

Co-authored-by: Benjamin Canac <canacb1@gmail.com>

src/theme/collapsible.ts

@@ -1,6 +1,6 @@
 export default {
   slots: {
-    root: 'flex flex-col gap-2',
+    root: '',
     content: 'data-[state=open]:animate-[collapsible-down_200ms_ease-out] data-[state=closed]:animate-[collapsible-up_200ms_ease-out] overflow-hidden'
   }
 }