Move char8_t to fmt/core.h

include/fmt/format.h

@@ -394,14 +394,6 @@ void buffer<T>::append(const U* begin, const U* end) {
 }
 }  // namespace internal
 
-// C++20 feature test, since r346892 Clang considers char8_t a fundamental
-// type in this mode. If this is the case __cpp_char8_t will be defined.
-#if !defined(__cpp_char8_t)
-// A UTF-8 code unit type.
-enum char8_t : unsigned char {};
-#endif
-template <> struct is_char<char8_t> : std::true_type {};
-
 // A UTF-8 string view.
 class u8string_view : public basic_string_view<char8_t> {
  public: