C++ named requirements: BasicFormatter (since C++20)

BasicFormatter is a type that abstracts formatting operations for a given formatting argument type and character type. Specializations of std::formatter are required to meet the requirements of BasicFormatter.

# Defect reports

DRApplied toBehavior as publishedCorrect behavior
LWG 3892C++20the value of pc.begin() was unclear if format-spec is not presentmade clear