feat(nuxt): add ssr route rule to enable SPA mode

[danielroe]

🔗 Linked issue

https://github.com/nuxt/framework/discussions/560

❓ 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 like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This change adds a ssr route rule to easily enable SPA mode for a route or group of routes:

Usage

export default defineNuxtConfig({
  nitro: {
    routes: {
      '/test': { ssr: false }
    }
  }
})

When unjs/nitro#538 merges (or in another PR implementing something like unjs/nitro@da12900), we can also refactor this to depend on event.context.routeOptions rather than having to construct a fresh matcher within the nuxt renderer.

📝 Checklist

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

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