Fix coercion types

[tmcw]

• Fixes Date coercion isn't supported by TypeScript types #682

This fixes the ability for fast-json-stringify to coerce Dates into strings and BigInts into numbers. These type tests weren't having any effect before: when schemas are declared like

// Number schemas
const schema1: Schema = {
    type: 'number'
}
const schema2: Schema = {
    type: 'integer'
}

build(schema1)(25)
build(schema2)(-5)

schema1 and schema2 are typed simply Schema, so none of the nice type inference is triggered and you can pass any argument to the method, like

build(schema1)({ xyz: 1 })

And there will be no type error. This adds some duplication because the two options that I know of are either adding as const to the end of each schema declaraction, or using a literal. as const doesn't work in this situation because it makes the type readonly, which then isn't accepted by build().

This disables a bunch of tests here: https://github.com/fastify/fast-json-stringify/pull/683/files#diff-72016aeffe80c3b259bbbf7e92a664cd2bcf49c2b79597740b4dcd8e430107bdR161 which are breaking for a different reason - the type declaration itself has always, afaict, been wrong, and there isn't an easy way to fix it. This PR just uncovered that pre-existing issue.

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

[tmcw]

Thanks! Is there additional process for getting this merged?

[gurgunday]

lgtm

I still refrain from merging type PRs 😄, so @Uzlopak will do it maybe?