Reintroduce constexpr fmt::formatted_size for C++20

[phprus]

Fix for #4102

FMT_DETAIL_EXPLICIT_INSTANTIATIONS is need to fix visual studio 2019 error:

D:\a\fmt\fmt\include\fmt\base.h(1198,21): error C2672: 'fmt::v11::detail::buffer<T>::append': no matching overloaded function found [D:\a\fmt\build\fmt.vcxproj]
...
D:\a\fmt\fmt\include\fmt\base.h(1198,1): error C2893: Failed to specialize function template 'void fmt::v11::detail::buffer<T>::append(const U *,const U *)' [D:\a\fmt\build\fmt.vcxproj]
          with
          [
              T=char
          ]
D:\a\fmt\fmt\include\fmt\base.h(929): message : see declaration of 'fmt::v11::detail::buffer<T>::append' [D:\a\fmt\build\fmt.vcxproj]
          with
          [
              T=char
          ]
D:\a\fmt\fmt\include\fmt\base.h(1198,1): message : With the following template arguments: [D:\a\fmt\build\fmt.vcxproj]
D:\a\fmt\fmt\include\fmt\base.h(1198,1): message : 'U=Char' [D:\a\fmt\build\fmt.vcxproj]

https://github.com/phprus/fmt/actions/runs/10221349236/job/28283689340

[vitaut]

This looks weird and probably a (benign) ODR violation. Maybe we should disable FMT_CONSTEXPR20 completely on this version of MSVC.

[phprus]

Visual studio 2019 supports C++20 and I think disabling FMT_CONSTEXPR20 for it is a bad idea.
I suggest disable FMT_CONSTEXPR20 only for fmt::formatted_size (implemented in new commit).

But, I don't see any ODR violation here, since the function body is the same.
If ODR is violated after PR, it must be violated without it due to explicit instantiation

[vitaut]

I think the new version is cleaner, let's go with it.

[phprus]

Another solution, but it will break library ABI: remove explicit instantiations for buffer<>::append.

Version with disabled FMT_CONSTEXPR20 for fmt::formatted_size on Visual studio 2019 is ready for review.

[vitaut]

Merged, thanks.