add to SimpleFaker class constructor optional defaultRefDate (like seed)

[Ghost-str]

It allows you to use a library like this when creating a custom instance:

import { Faker, ru, en, base } from '@faker-js/faker';

const faker = new Faker({
  locale: [ru, en, base],
  seed: 1234567890,
  defaultRefDate: '2026-04-01T01:01:01.900Z'
});

[ST-DDT]

This might conflict with/get resolved with #2838

• feat: introduce FakerCore #2838

As it allows passing a config object to the constructor.

new Faker({
  config: {
    defaultRefDate: () => new Date(),
  },
})

This also allows future options to be specified, if ever added.