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/ui_api.cpp

@@ -630,6 +630,12 @@ namespace ExtUI {
     feedrate_percentage = clamp(value, 10, 500);
   }
 
+  void setUserConfirmed(void) {
+    #if HAS_RESUME_CONTINUE
+      wait_for_user = false;
+    #endif
+  }
+
   void printFile(const char *filename) {
     IFSD(card.openAndPrintFile(filename), NOOP);
   }