Fix typings for http methods shortcuts on fastify reply

[g0di]

Fix typings for HTTP methods shortcuts added on the FastifyReply object.

Those signatures were indicating a void return while the implementations were returning the reply itself.

fixes #133).

I've only updated the typings tests accordingly because the implementation tests were already ensuring that those shortcuts were returning the fastify reply object.

Also note that I've moved the following type from httpError.d.ts to index.d.ts

type HttpErrorReplys = {
  getHttpError: (code: HttpErrorCodes, message?: string) => FastifyReply;
} & Record<HttpErrorNames, (msg?: string) => FastifyReply>;

The reason is that I think the lib folder may not contain code related to fastify itself. It made more sense to me to put that type as close as possible to the concerned FastifyReply interface.

Checklist

• run npm run test and npm run benchmark (there is no benchmark script)
• tests and/or benchmarks are included
• documentation is changed or added (no need to update the doc)
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm