lack of strictSchema in ajv options results in silent typos

### Prerequisites

- [X] I have written a descriptive issue title
- [X] I have searched existing issues to ensure the issue has not already been raised

### Issue

`strictSchema` is [set to false in the ajv options](https://github.com/fastify/fast-json-stringify/blob/9b30d17d87c11628944bc9f33c491f6df6765653/lib/validator.js#L12) (and overrides any user-provided `ajv:` options to the constructor, so it cannot be set to true by the user either).

This makes it very easily to add a schema with an error that is silently ignored.

In my case I made a typo with `additionalProperties` in a fastify response schema, which meant that additional properties were not removed from a response on serialisation that should have been.

It would seem like having strictSchema:true would be a better default? (which would be a breaking change needing a major version I suppose), or at least allow the user to set this.

I tested setting `strictSchema:true` and running the tests, and the only additional keyword used internally seems to be `kind`[1],[2], which is only used in tests.

[1]: https://github.com/fastify/fast-json-stringify/blob/9b30d17d87c11628944bc9f33c491f6df6765653/test/anyof.test.js#L206
[2]: https://github.com/fastify/fast-json-stringify/blob/9b30d17d87c11628944bc9f33c491f6df6765653/test/array.test.js#L371