Expand the functionality of the linked list

[Smidqe]

At the moment all iterations on playlist have to be done manually every time something is needed, creating unnecessary code duplication.

This pull request introduces methods on the linked list to unify these operations (ideally playlist would be an array).

[nelsonlaquet]

I would suggest making sure that this method has a verb in the name (unless this name comes from a standard somewhere?). It's fairly clear what it does at the callsites, but it took a second to figure out what it did when I first saw this line. Something like getMultipleByIndex or something.

super-nitpicky optimization suggestions:

1. construct the result array in the loop to remove the last line:

const result = [];
for (...) {
  if (stuff) {
    result.push(...);
  }
}
return result;

2. use a Set instead of Map to save a few array allocations
3. calculate the max index at the same time as populating the Set

[nelsonlaquet]

nit: I'm guessing this should be findIndex if you're wanting to keep aligned with the same naming as the array functions

[Atte]

While you're at it, maybe the linked list should be turned into a class instead of doing the prototype shenanigans?