Add support for Typescript types

[sagarsys]

Currently, if I try to use this module with the fastify-cli Typescript template, I get the following error:

TS7016: Could not find a declaration file for module 'fastify-prettier'. '/node_modules/fastify-prettier/src/index.js' implicitly has an 'any' type.   Try npm i --save-dev @types/fastify-prettier if it exists or add a new declaration (.d.ts) file containing declare module 'fastify-prettier';

It would be good to have the official types for this module.

[hsynlms]

@sagarsys a PR about this would be highly welcomed.