Add Stringified type

[kainiedziela]

Fixes #87

Create a type with all of the inputed types keys transformed into strings.

Use case: changing an interfaces values to strings in order to use it as a form model.

Example

import {Stringified} from 'type-fest';

type Car {
	model: string;
	speed: number;
}

const carForm: Stringified<Car> = {
	model: 'Foo',
	speed: '101'
};