Add a conditional compilation directive to be compatible with C++26

[vspefs]

cppfront now fails to compile under C++26 for a simple repetition of definition (documented beforehand) with C++26 standard library.

The comments in source/common.h documented:

//  In keep trying to write string+string_view, and it ought to Just Work without
//  the current workarounds. Not having that is a minor impediment to using safe
//  and efficient string_views, which we should be encouraging. So for my own use
//  and to remove that minor impediment to writing safe and efficient code, I'm
//  just going to add this until we get P2591 in C++26(?) -- See: wg21.link/p2591
//

and now P2591 has been accepted and assigned a feature test macro, we just have to conditionally disable it with FTM (__cpp_lib_string_view). Then we can have cppfront compiled and running under C++26.

[hsutter]

Thanks!

This is almost editorial so I won't ask for a CLA, but if you intend to submit any future PRs please email me (first dot last at gmail) with your full name and I'll send you a one-time CLA to sign that will cover your future PRs.