chore: add `.gitattributes` file

[Fdawgs]

Noticed line endings changed with https://github.com/fastify/fastify-jwt/pull/302 and realised this doesn't have a .gitattributes file.

Seems to be missing from other org repos as well, so happy to add them to the missing ones: https://github.com/search?q=org%3Afastify%20.gitattributes&type=code

Checklist

• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct

[Uzlopak]

@Fdawgs https://twitter.com/anothergalvez/status/1711099349030080591