[fullstack] integrate to nitro vite plugin

[hi-ogawa]

• Integrate as external plugin feat: add vue-router-ssr example with SSR assets API nitrojs/vite-examples#5
  • works as ssr environment
  • works as nitro environment
    • some build pipeline complication.
    • dynamic client entry is not possible (but this is acceptable since it only requires configuring main client entry).
    • we just need to emit asset manifest after nitro.
    • actually it's possible by ordering fullstack -> nitro
• integrate directly as experimental option