Flags for MarlinSerial instance features (#21318)

Marlin/src/feature/meatpack.h

@@ -142,6 +142,8 @@ struct MeatpackSerial : public SerialBase <MeatpackSerial < SerialT >> {
   // Existing instances implement Arduino's operator bool, so use that if it's available
   bool connected()                    { return Private::HasMember_connected<SerialT>::value ? CALL_IF_EXISTS(bool, &out, connected) : (bool)out; }
   void flushTX()                      { CALL_IF_EXISTS(void, &out, flushTX); }
+  SerialFeature features(serial_index_t index) const  { return SerialFeature::MeatPack | CALL_IF_EXISTS(SerialFeature, &out, features, index);  }
+
 
   int available(serial_index_t index) {
     if (charCount) return charCount;          // The buffer still has data