int64 validation is incorrect

https://github.com/ajv-validator/ajv-formats/blob/4dd65447575b35d0187c6b125383366969e6267e/src/formats.ts#L252

> // JSON and javascript max Int is 2**53, so any int that passes isInteger is valid for Int64

The second part of the sentence does not follow from the first. Neither JS nor JSON do not throw on overflows.

Consider this:

```
> Number.isInteger(JSON.parse('' + (2 ** 65)))
true
```