Add a peer dependency on `fastify` to fix TypeScript types

[walkerburgin]

The TypeScript typings in types/index.d.ts import explicitly from fastify without this package taking a dependency on fastify. If consumers are using a strict package manager like PNPM or Yarn (Berry), the typings are currently broken. This PR adds a peer dependency on fastify to fix TypeScript typings.

Checklist

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