Have as=a for Responsive Nav Link (#1543)

This why like DropdownLink we can link to non inertia pages for example 

```
                            <ResponsiveNavLink
                                as="a"
                                :href="route('filament.admin.pages.dashboard')" :active="route().current('filament.admin.pages.dashboard')">
                                Admin Area
                            </ResponsiveNavLink>
```

Author: alnutile

stubs/inertia/resources/js/Components/ResponsiveNavLink.vue

@@ -21,6 +21,10 @@ const classes = computed(() => {
             <slot />
         </button>
 
+        <a v-else-if="as == 'a'" :class="classes" class="w-full text-start" :href="href">
+            <slot />
+        </a>
+
         <Link v-else :href="href" :class="classes">
             <slot />
         </Link>