Support nodenext

[jonnydgreen]

Closes: #1047

• I have taken the code as is (to de-risk) and just moved the root level types into the declared namespace, other key bits are detailed as follows:

type Mercurius = typeof mercurius

declare namespace mercurius {
  // ...

  export const mercurius: Mercurius
  export { mercurius as default }
}

declare module "fastify" {
  // ...
}

declare function mercurius
  (
    instance: FastifyInstance,
    opts: mercurius.MercuriusOptions
  ): void;

export = mercurius;

• All the types tests pass and have added some other tests where needed
• Have taken some inspiration from the fastify repo and other plugins for consistency
• Have also tested against this repo and all appears to be okay: https://github.com/iamchathu/mercurius-ts-example

[jonnydgreen]

Opening an early PR to get your initial thoughts! Essentially, it's just a lift and shift - but keen to get feedback :)

[jonnydgreen]

Cool - thanks! Just ran some final testing and all good to go :)