feat(nuxt, kit): add shortcuts to register custom server handlers

[pi0]

🔗 Linked issue

Resolvesnuxt/nuxt#13892

❓ 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 PR marks serverMiddleware option and addServerMiddleware as deprecated and introduces two new serverHandlers and devServerHandlers as shortcut options to register custom server handlers + kit utilities addServerHandler and addDevServerHandler.

📝 Checklist

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

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