Fix assert types to accurately reflect implementation

[englercj]

Improve the assert typings to match node's assert module. This informs Typescript to narrow the type of the parameters, and allows values that are not strictly boolean. Also the previous typings indicated a string return value, but that doesn't seem to match the JS implementation.

Ideally this code should compile correctly, and correctly inform Typescript about the assertion being made:

fastify.assert(obj, 404);
fastify.assert(obj.someValue, 422);

Currently this doesn't compile correctly for various reasons, which are fixed in this PR.

note: The checklist says to run "npm run benchmark" but no such script exists.

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

[RafaelGSS]

LGTM.