Improve typing of Difference interface

[Schrubitteflau]

Create 3 specialized Difference interfaces which allows us to have some type-hinting when the value of the property type is tested :

const differences: Difference[] = diff({}, {});

for (const difference of differences) {
	switch (difference.type) {
		case "CREATE":
			console.log(difference); // type-hint : DifferenceCreate
			difference.oldValue; // Property 'oldValue' does not exist on type 'DifferenceCreate'
			break;
		case "REMOVE":
			console.log(difference); // type-hint : DifferenceRemove
			difference.value; // Property 'value' does not exist on type 'DifferenceRemove'
			break;
		case "CHANGE":
			console.log(difference); // type-hint : DifferenceChange
			difference.value // OK
			difference.oldValue // OK
			break;
	}
}

[AsyncBanana]

Looks good! It should help the development experience with TypeScript. Thanks for the PR!