Add named export to type definitions

[wight554]

• similar to Make TS types more consistent with other fastify plugins fastify-cookie#97

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

[wight554]

Main point of having this is that typescript looses typings when using default import of CJS module with tsconfig module: "nodenext" setting from TS 4.7:

import fastifyCookie
Argument of type 'typeof import("/.../node_modules/fastify-cookie/plugin")' is not assignable to parameter of type 'FastifyPluginCallback<any, Server> | FastifyPluginAsync<any, Server> | Promise<{ default: FastifyPluginCallback<any, Server>; }> | Promise<...>'.ts(2345)

More info here: microsoft/TypeScript#48845

[climba03003]

LGTM.

[mcollina]

lgtm