Add backport of std::expected instead of Result.hpp #91

Is your feature request related to a problem? Please describe.
Result.hpp is developer defined code for std::expected functionality. std::expected inroduced in C++23, but project standard is C++17.

Describe the solution you'd like
Replace Result.hpp by std::expected backport to C++17.

Describe alternatives you've considered
https://github.com/zeus-cpp/expected - good solution for C++17

Additional context
Need to be sure, that file can be placed in project like mustache.hpp