Add type guard to 'is' and 'is.not' methods

[inhibitor1217]

As each instances are defined as branded type of the extended enum (#7), type guards should be supported.

enum _Fruit { Apple, Orange }

class Fruit extends extend<typeof _Fruit, _Fruit>(_Fruit) {}

declare const f: Fruit;

if (f === Fruit.Apple) {
  type T = typeof f; // should be Fruit.Apple, it is not working now regardless of type guards
}

if (f.is(Fruit.Apple)) {
  type T = typeof f; // Fruit.Apple
}

if (f.is.not(fruit.Apple)) {
  type T = typeof f; // Exclude<F, Fruit.Apple> or something similar
}

[inhibitor1217]

Apparently, type guards by nested property is not supported in current version of TypeScript. (related pull request - microsoft/TypeScript#38839)

Therefore, providing a type guard for is.not is not possible right now.

We propose an alternative to use isNot as an alias of is.not. isNot lives as a direct property of ExtendedEnum, hence it can be used for type narrowing.

is.not will continue to be supported, but type guards will not be provided.