Why do we create an async function and void it here? Why not keep the for loop as it was before?

@myarmolinsky the whole point of this PR is to have this function return the stream w/o having to wait for the whole stream to be constructed (which is what the async-await part does). I'm still (like before) using async-await in here

• b/c I'm using await for the param to the eolConverter (since I wasn't confident how I could correctly change it to be stream based)
• for convenience, since it made it easier to (a)wait till all files are added, before we call await preFinalizeCallback & pack.finalize()

So the function returns right away the stream reference, and the void allows us to keep the async-await part working in the background and asynchronously add all files in the stream in order and call all required other functions/callbacks.

If we convert eolConverter to be stream based in a separate PR, we can then get rid of the void-async-await, and replace that by adding event listeners on every operation (tbh, I find the async-await code easier to read).