gh-51338: Fix shlex to preserve newlines after comments in POSIX mode

[kamilturek]

According to POSIX standard, newlines are NOT part of comments. Previously, shlex incorrectly consumed newlines when skipping comments, causing them to be lost even when excluded from the whitespace attribute.

If the current character is a '#', it and all subsequent characters up to, but excluding, the next <newline> shall be discarded as a comment. The <newline> that ends the line is not considered part of the comment.

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_03

• Issue: shlex behaves unexpected if newlines are not whitespace #51338