Send notifications to ExtUI for M0/M1 (#13344)

- Send notifications to ExtUI for M0/M1

- wait_for_user can be non-volatile (not changed by interrupt)
  C / C++ compilers don't optimize away reads of non-volatile variables when a function call is used between accesses, because *any* variable could be changed by the function call. Since `wait_for_user` can't be changed without a function call, it should be non-volatile so the compiler can optimize away cases where it is read more than once without an intervening function call.

Marlin/src/lcd/extensible_ui/lib/example.cpp

@@ -46,7 +46,7 @@ namespace ExtUI {
      */
   }
   void onIdle() {}
-  void onPrinterKilled(const char* msg) {}
+  void onPrinterKilled(PGM_P const msg) {}
   void onMediaInserted() {};
   void onMediaError() {};
   void onMediaRemoved() {};
@@ -55,6 +55,7 @@ namespace ExtUI {
   void onPrintTimerPaused() {}
   void onPrintTimerStopped() {}
   void onFilamentRunout() {}
+  void onUserConfirmRequired(const char * const msg) {}
   void onStatusChanged(const char * const msg) {}
   void onFactoryReset() {}
   void onLoadSettings() {}