[upstream] get_language and get_stdopt from xeus-cpp

[mcbarton]

In compiler-research/xeus-cpp#461 and compiler-research/xeus-cpp#462 get_stdopt() was adapted to make use of Cpp::GetLanguageStandard and get_language was created making use of Cpp::GetLanguage . These CppInterOp APIs were originally created as enums, while xeus-cpp requires the language and standard as a string. get_stdopt and get_language should be upstreamed to CppInterOp so that the possible returned string values keep upto date with the possible enum options.