neverthrow feat: fork()

https://github.com/supermacro/neverthrow/issues/594

fork() allows you to handle Ok<.> and Err<.> at once. This is similar to Promise:then(a, b), which is not the same as Promise .then(a).catch(b) nor .catch(b).then(a) (see image https://stackoverflow.com/a/24663315)

While match() could already be used to achieve this for Result<A, B> by simply returning a Result<T, U> in either branch, this approach falls apart for ResultAsync, as match() makes the return type to be a promise. Fork() is especially useful in this case, as it allows you to chain methods available on ResultAsync.

// We can chain on .match() by returning a Result<.>
const matchExample1 = ok(123)
  .match(
    (x) => ok(x + 321),
    (y) => err(y + 444),
  )
  .map((x) => x - 321)

// We cannot do the same when using .match() on ResultAsync<.>
const asyncMatchExample1 = okAsync(123)
  .match(
    (x) => okAsync(x + 321),
    (y) => errAsync(y + 444),
  )
  // TS2339: Property map does not exist on type
  // Promise<ResultAsync<number, never> | ResultAsync<never, number>>
  .map((x) => x - 321)

// To be able to chain on ResultAsync<.>:match we must currently use fromSafePromise(.).andThen(x => x)
const asyncMatchExample1Fixed = fromSafePromise(
  okAsync(123).match(
    (x) => okAsync(x + 321),
    (y) => errAsync(y + 444),
  ),
)
  // flatten
  .andThen((x) => x)
  .map((x) => x - 321)

// .fork() works just like .match() on Result<.> (except its handlers must return a Result<.>)
const forkExample1 = ok(123)
  .fork(
    (x) => ok(x + 321),
    (y) => err(y + 444),
  )
  .map((x) => x - 321)

// We no longer need fromSafePromise(.).andThen(x=>x) when matching on ResultAsync
const asyncForkExample1 = okAsync(123)
  .fork(
    (x) => ok(x + 321),
    (y) => err(y + 444),
  )
  .map((x) => x - 321)