P2216R3 `std::format` Improvements

[StephanTLavavej]

P2216R3 std::format Improvements

Feature-test macro:
#define __cpp_lib_format INCREASED_VALUE_TO_BE_DETERMINED

At the June 2021 virtual plenary meeting, this was accepted as a defect report for C++20, which means that this paper applies retroactively to the C++20 Standard.

[fsb4000]

I saw @miscco wrote about marking as blocked #1447
I think we should also mark as blocked this issue because: #1882