Add Cap:ARCS

Marlin/src/gcode/host/M115.cpp

     // MOTION_MODES (M80-M89)
     cap_line(PSTR("MOTION_MODES"), ENABLED(GCODE_MOTION_MODES));
 
+    // ARC_SUPPORT (G2-G3)
+    cap_line(PSTR("ARCS"), ENABLED(ARC_SUPPORT));
+
     // BABYSTEPPING (M290)
     cap_line(PSTR("BABYSTEPPING"), ENABLED(BABYSTEPPING));