Fix implementation of to_utf8 and to_utf16

[StackDoubleFlow]

The current implementation of these functions do not properly convert between the unicode encodings and have caused bugs in the past. This new implementation makes use of std::codecvt and std::wstring_convert.

Although these specializations of codecvt are now deprecated in c++20, they will still work until there is a suitable replacement in the standard library, and as far as I know, such a thing does not exist yet.

It is recommended to use a proper library for this task. il2cpp has built in methods of performing this conversion, so it may be worth looking into that in the future.

[sc2ad]

Consider using the string conversion functions available in typedefs-string.hpp, those functions perform the correct conversion and also are not deprecated. I do agree that the current implementations of to_utf8 and to_utf16 need some work, though, so I appreciate the step towards that.