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'
}
}
}
```

Look at the existing error handling in fastify-micro, likely in a plugin or middleware. Integrate the provided RFC7807 schema and error class, ensuring compatibility with fastify.sensible. Test by creating errors and verifying the response format matches the RFC.

Written by the indexing model from the issue text.