`date` and `date-time` do not support `formatMaximum` validation for years >=10000 🤷‍♂️

When you create a `Date` object in UTC timezone in JavaScript as follows

```js
new Date(10000, null)
```

and convert it to ISO string, you get `+010000-01-01T00:00:00.000Z` (notice the leading `+0` string).

This seems to [follow the ISO 8601 standard](https://en.wikipedia.org/wiki/ISO_8601#Years).

This breaks the `formatMaximum` (and possibly also `formatMinimum`) validation because the dates are compared as plain strings, ie. eventually you get to a point where your compare `"2018"` and `"+010000"` and as far as JS strings are concerned, `2018` is greater, even though the intention was to compare year 2018 and year 10000.

Here, the ISO string is split into year component:

https://github.com/epoberezkin/ajv-keywords/blob/5f9eccc2e2b95745f659ef1abf5488f70da496f2/keywords/_formatLimit.js#L83-L87

And here is the actual string comparison of the two dates:

https://github.com/epoberezkin/ajv-keywords/blob/5f9eccc2e2b95745f659ef1abf5488f70da496f2/keywords/_formatLimit.js#L62-L67