Fallback to fmtlib When std::format Unavailable

[mfisher31]

Solution

Currently there is workaround code in lui::String that checks std::format availability. If not present, then formatting is don't with std::to_string which is a performance hit.

Solution

fallback to fmtlib/fmt. This can be a header-only library, supports CMake, and I believe was rolled in to c++ itself.

Add a file lui/format.hpp that will use std::format if availalbe, otherwise use fmt::format.

Fallback Logic

Details needed.