Rmorozov/fix std20

[rmorozov]

Make jinja2cpp compilable under clang 12 with -std=c++20
the problem arose from the fact that libfmt switched to consteval check of format
string(that is great, but makes us obliged to provide format_to with
constexpr format_string or fmt::runtime overload)

unfortunatelly I couldn't make it work with constexpr std::string
so multistringliteral returns std::basic_string_view except for cases
where it needs std::string

fix https://github.com/jinja2cpp/Jinja2Cpp/issues/213

[rmorozov]

unfortunately PR messed up with my experiments with clang-tidy=(

Sorry for that