Add card.longest_filename method

Marlin/src/sd/cardreader.h

@@ -144,6 +144,8 @@ public:
     }
   #endif
 
+  FORCE_INLINE char* longest_filename() { return longFilename[0] ? longFilename : filename; }
+
 public:
   bool saving, logging, sdprinting, cardOK, filenameIsDir;
   char filename[FILENAME_LENGTH], longFilename[LONG_FILENAME_LENGTH];