Fix line numbers for multiple files

[SimonCW]

This fixes the differences to the cat implementation for multiple files. See #9

Unfortunately, I haven't found a fix while still using enumerate. So, this kind of deviates from the text in the book.

[kyclark]

This is a different implementation of cat that numbers all lines without resetting for each file. The version I was implementing starts over at 1 with each file. There's nothing wrong with this PR, and it fixes the test input nicely, but I was implementing a different program.

[rdlu]

I found the same issue using the mk-outs.sh on my system. It generates line number 1..13.

I'm using Fedora 38 with cat (GNU coreutils) 9.1.

Another solution is using bat, a cat replacement in rust. But it doesn't support -b, only -n.

So I think this is just the cat on MacOS that presents this behavior. There's a bsdcat on Fedora repo, but it doesn't supports any flags.