WG21-P3142R0 Printing Blank Lines With println()

No feature-test macro is mentioned.

• This feature is really simple.
• It doesn't introduce any new identifiers, so there's no real potential for conflict.
• Nobody has finished C++23 yet.

Although this wasn't formally accepted as a Defect Report, I've reached out to libstdc++ and libc++'s maintainers, and everyone agrees that we should implement this "downlevel" in C++23 mode.