Update Italian language (#12344)

Marlin/src/lcd/language/language_it.h

 #define MSG_LEVEL_CORNERS                   _UxGT("Livella spigoli")
 #define MSG_NEXT_CORNER                     _UxGT("Prossimo spigolo")
 #define MSG_EDITING_STOPPED                 _UxGT("Modif. Mesh Fermata")
+#define MSG_MESH_X                          _UxGT("Indice X")
+#define MSG_MESH_Y                          _UxGT("Indice Y")
+#define MSG_MESH_EDIT_Z                     _UxGT("Valore di Z")
 #define MSG_USER_MENU                       _UxGT("Comandi personaliz.")
 #define MSG_UBL_DOING_G29                   _UxGT("G29 in corso")
 #define MSG_UBL_UNHOMED                     _UxGT("Home XYZ prima")