Fix SerializerCompiler type

[remidewitte]

PR for #47 .

A reflexion is that RouteDefinition could rather be because in the context of the compiler I think the compiler would never be called if the schema is undefined and most of the time url, method and httpStatus are not used :

export type RouteDefinition = {
    method?: string;
    url?: string;
    httpStatus?: string;
    schema: unknown;
  }

And if used with reply.getSerializationFunction or reply.compileSerializationSchema they can be undefined :

.getSerializationFunction(schema | httpStatus, [contentType]) - Returns the serialization function for the specified schema or http status, if any of either are set.
.compileSerializationSchema(schema, [httpStatus], [contentType]) - Compiles the specified schema and returns a serialization function using the default (or customized) SerializerCompiler. The optional httpStatus is forwarded to the SerializerCompiler if provided, default to undefined.

What do you think ?

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

[mcollina]

lgtm

[Uzlopak]

I adapted the linting issues, but you decide