RFC-7807 for errors

In combination with `fastify.sensible`, make it easy to send well-formatted errors to clients, with actionable details for better DX.

Specification: https://tools.ietf.org/html/rfc7807

Copy-pasta from another project:
```ts
// https://tools.ietf.org/html/rfc7807

import { STATUS_CODES } from 'node:http'
import { z } from 'zod'

export const rfc7807Schema = z.object({
status: z
.number()
.int()
.refine(code => code in STATUS_CODES),
type: z.string().url().optional(), // Technically a URI, but let's not split hairs
title: z.string(),
detail: z.string().optional(),
instance: z.string().url().optional()
})

export type RFC7807 = z.TypeOf

export class RFC7807Error extends Error {
readonly rfc7807: RFC7807

constructor(args: RFC7807) {
super(
`${STATUS_CODES[args.status]}: ${args.title}${
args.detail ? ` (${args.detail})` : ''
}`
)
this.name = 'RFC7807Error'
this.rfc7807 = {
...args,
type: args.type ?? 'about:blank'
}
}
}
```