Add C parameter to M27 to get the current filename

In answer to #10001

Add an option to retrieve the currently open file name (long filename if possible).

Marlin/cardreader.h

@@ -61,6 +61,7 @@ public:
   );
   void getStatus();
   void printingHasFinished();
+  void printFilename();
 
   #if ENABLED(LONG_FILENAME_HOST_SUPPORT)
     void printLongPath(char *path);