Change type to `std::basic_string<char16_t>`

Changes the type to std::basic_string<char16_t> according to https://en.cppreference.com/w/cpp/string/basic_string,

the deprecated char_traits base template has been removed in LLVM 19.

char16_t is a typedef to unsigned short on musl libc and __uint_least16_t on glibc systems.

Fixes #1462 (closed)