use namespace merging for nodenext

[Uzlopak]

@climba03003 Regarding namespace merging.

It doesnt work. I assume because of

If you have an import or an export at the root level of a TypeScript file then it creates a local scope within that file.

https://jorgedacostaza.gitbook.io/typescript-pt/project/modules

@fastify/typescript
Any suggestion to fix this to use namespace merging across files?

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Uzlopak]

@