Adds char8_t as a byte-like type, which in turn enables C++20 std::u8string and std::u8string_view (aliases for std::basic_string<char8_t> and std::basic_string_view<char8_t>) objects to be passed in as spans. Right now, simdutf fails to compile if users pass these objects.

cppreference states (emphasis mine):

char8_t — type for UTF-8 character representation, required to be large enough to represent any UTF-8 code unit (8 bits). It has the same size, signedness, and alignment as unsigned char (and therefore, the same size and alignment as char and signed char), but is a distinct type. (since C++20)

This is enabled in >=C++20 only, and it won't affect older versions.