✨ M115 flag EXTENDED_M20 (#22941)

Marlin/src/gcode/host/M115.cpp

@@ -143,6 +143,9 @@ void GcodeSuite::M115() {
     // LONG_FILENAME_HOST_SUPPORT (M33)
     cap_line(F("LONG_FILENAME"), ENABLED(LONG_FILENAME_HOST_SUPPORT));
 
+    // EXTENDED_M20 (M20 L)
+    cap_line(F("EXTENDED_M20"), ENABLED(LONG_FILENAME_HOST_SUPPORT));
+
     // THERMAL_PROTECTION
     cap_line(F("THERMAL_PROTECTION"), ENABLED(THERMALLY_SAFE));