feat(Select/SelectMenu): Add group-label and separator slots to Select and SelectMenu

[OpenSnack]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Adds the following optional slots to the Select and SelectMenu components:

• group-label: Replaces the default content when an item with the label type is rendered.
• separator: Replaces the default content when an item with the separator type is rendered.

This allows you to further customize the content before/between item groups, which is possible in Reka UI but not currently possible in Nuxt UI. Both slots provide the current item and index, as with the other slots that already exist. The group-label slot also provides groupIndex.

<USelect :items="items">
  <template #group-label="{ item, index, groupIndex }">
    <!-- content -->
  </template>
  <template #separator="{ item, index }">
    <!-- content -->
  </template>
</USelect>

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

(As I understand it, the relevant documentation for this change is updated automatically.)

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/ui@4248

commit: b4a3633