Add polyfill for C++26's <debugging>

[calcmogul]

Per https://en.cppreference.com/w/cpp/header/debugging:

• std::breakpoint() pauses the running program when called
• std::breakpoint_if_debugging() calls std::breakpoint() if std::is_debugger_present() returns true
• std::is_debugger_present() checks whether a program is running under the control of a debugger

Here's a reference implementation from P2546 that we can import: https://github.com/grafikrobot/debugging