libstdc++: Implement "Printing blank lines with println" for C++23

This was recently approved for C++26 at the Tokyo meeting. As suggested
by Stephan T. Lavavej, I'm defining it as an extension for C++23 mode
(when std::print and std::prinln were first added) rather than as a new
C++26 feature. Both MSVC and libc++ have agreed to do this too.

libstdc++-v3/ChangeLog:

	* include/std/ostream (println(ostream&)): Define new overload.
	* include/std/print (println(FILE*), println()): Likewise.
	* testsuite/27_io/basic_ostream/print/2.cc: New test.
	* testsuite/27_io/print/1.cc: Remove unused header.
	* testsuite/27_io/print/3.cc: New test.