Allow specifying the type of params

[Dimrok]

Rational

Like #94 with Date, I faced a problem with some "complex" types I wanted to pass as params.

The Pull Request allows one to specify the expected type of params (initially DeepPartial<T>) so we can easily use types such as luxon's DateTime without altering the library (unlike #94).

Note: For backward compatibility, P (type of params) was of course defaulted to DeepPartial<T>.

Example

Before

The library used to prevent this:

import { DateTime } from 'luxon';

type Event = {
  id: string;
  createdAt: DateTime;
}

export class EventFactory extends Factory<Event> {}

export const eventFactory = EventFactory.define(({ params }) => {
  const createdAt = createdAt ?? DateTime.fromJSDate(faker.date.soon());

  return {
    id: '1',
    createdAt,
  };
});

params#createdAt was a DeepPartial<DateTime>, which prevent the pattern const createdAt = createdAt ?? DateTime.fromJSDate(faker.date.soon());

After

import { DateTime } from 'luxon';

type Event = {
  id: string;
  createdAt: DateTime;
}

type EventParams = DeepPartial<Event> & {
  createdAt?: DateTime;
};  

export class EventFactory extends Factory<Event, {}, Event, EventParams> {}

export const eventFactory = EventFactory.define(({ params }) => {
  const createdAt = createdAt ?? DateTime.fromJSDate(faker.date.soon());

  return {
    id: '1',
    createdAt,
  };
});

[filipkis]

It would be really great to get this merged as we're stumbling on this issue now that I'm running out of ideas on how to solve since we use rather complex types.

[Mange]

This looks very sensible to me.