Improve/fix D23 for M2.5/S printers

- Move D23 into it's own function inside Dcodes
- Correctly include a break in the switch statement
- Show the dumper status (enabled/disabled) after toggling
- Allow to generate an immediate dump via g-code using D23 E for
  symmetry with D20 E

Firmware/Dcodes.h

@@ -37,7 +37,8 @@ extern void dcode_22(); //D22 - Clear crash dump state
 
 #ifdef EMERGENCY_SERIAL_DUMP
 #include "xflash_dump.h"
-extern bool emergency_serial_dump;
+extern void dcode_23(); //D23 - Request/generate an online serial crash dump
+extern bool emergency_serial_dump; //emergency dump enabled flag
 extern void serial_dump_and_reset(dump_crash_reason);
 #endif