Support Nitro v3

[MickL]

Would be amazing to support Nitro v3 which has just been released as alpha :)

• Replace imports of nitropack with nitro
• No more auto imports (if used)
• Some functions from h3 are now deprecated in favor of web standards, e.g. getHeader(event, 'name') -> event.headers.get('name') (if used)
• Nitro v3 now is a Vite plugin. Do we benefit from that because nitro-test-utils is using Vitest?

Migration guide: https://v3.nitro.build/docs/migration