Fixes: Linux local files + last line edge case

[kona314]

Hi there, thanks for making the fastest and most efficient CSV-parsing Swift package out there!

I've added these improvements to my local copy.

• The first works around URL.AsyncBytes not being available on Linux, which was preventing me from using this to read local files on a server. I'm not pretending it's an elegant implementation, but it does work.
• The second addresses an edge case I ran into where (1) the last data line included missing values (i.e. comma delimiters next to each other) and (2) no trailing empty line. This condition caused it to return n-1 columns, which threw an error.