Take a first stab at async results

[jstasiak]

It's very common to see this pattern in our (Lune) code:

const thing: Promise<Result<T, E>> = ...
// Now we want to return an error if Err but if Ok we want to
// process it further with asynchronous code, so first await it:
const result = await thing

// Then manualy branch on the variant:
if (result.isErr()) {
    return result
}

// Only now we can process it:

const anotherThing: Promise<Result<U, E>> = asyncFunction(result.value)

// But wait! We want to process it with *another* function
(asyncAnotherFunction).
// That means another await/if/return/process block.
// ...

This PR attempts to make this whole thing much better and now the following will be possible:

return new AsyncResult(thing)
    .andThen(asyncFunction)
    .andThen(asyncAnotherFunction)

• Why a separate type? Because I didn't find a way to fit promises/asynchronous mappers in the existing API in a way that provides static type safety and works correctly at runtime.
• Why only AsyncResult and not AsyncOption? Both makes sense but AsyncResult is the biggest bang for the buck for us.
• Why only the two methods (andThen and map)? We need these the most, the rest can be added later (when/if we decide we need them).
• Why not add async versions of the methods directly to Result (asyncMap, asyncAndThen etc.)? I decided it would result in too much duplication.
• Why allow synchronous functions to be composed with AsyncResult? For convenience – once you're in the AsyncResult realm it'll be the easiests to just stay in it and compose as many things as possible before finally converting back to Result.

I already implemented more methods so I kept the implementations commented out just so
we don't have to do the same work again.

Part-of: https://linear.app/lune/issue/LUN-2855/add-async-support-to-ts-results-es

[rbruggem]

What is E here?

[jstasiak]

The type of the original error (if the result is an error (type E) then andThen() keeps it and doesn't call mapper.

[rbruggem]

Why is T required in AsyncResult<T | T2, ?

[jstasiak]

It's not, good spot. Whatever mapper() returns is our only possible success value type here.

Fixed.

[rbruggem]

Nice.

[raguiar9080]