std::u8string support (with utf8_iterator)

Now you can use std::u8string and std::u8string_view with CTRE! 🎉

std::optional<std::u8string> match(std::u8string_view subject) {
	if (auto res = ctre::match<"(\\p{emoji}+)">(subject)) {
		return res.get<1>().to_string();
	} else {
		return std::nullopt;
	}
}

Also you can use ctre::utf8_range to convert between unicode encodings:

std::u32string convert(std::u8string_view input) {
	auto r = ctre::utf8_range(input);
	return std::u32string(r.begin(), r.end());
}