[review][feature] Add C++26 constexpr formatting support

[mborland]

C++26 works in constexpr now
https://eel.is/c++draft/format#def:constexpr-enabled
Added with [P3391] I think.

SInce your types are already very constexpr-enabled,
IMO would be good to also add constexpr formatting.

Currently not working: https://godbolt.org/z/oac5fcara
I am assuming it's easy, like just missing constexpr on formatter::format(),
since adding -fimplicit-constexpr makes it work: https://godbolt.org/z/fhYPKWqGT

Also check {fmt} since they probably have it too