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/gcode/lcd/M0_M1.cpp

@@ -31,6 +31,10 @@
   #include "../../lcd/ultralcd.h"
 #endif
 
+#if ENABLED(EXTENSIBLE_UI)
+  #include "../../lcd/extensible_ui/ui_api.h"
+#endif
+
 #include "../../sd/cardreader.h"
 
 #if HAS_LEDS_OFF_FLAG
@@ -74,6 +78,10 @@ void GcodeSuite::M0_M1() {
       #endif
     }
 
+  #elif ENABLED(EXTENSIBLE_UI)
+
+    ExtUI::onUserConfirmRequired(has_message ? args : MSG_USERWAIT); // SRAM string
+
   #else
 
     if (has_message) {
@@ -97,6 +105,10 @@ void GcodeSuite::M0_M1() {
   else
     while (wait_for_user) idle();
 
+  #if ENABLED(EXTENSIBLE_UI)
+    ExtUI::onUserConfirmRequired(nullptr);
+  #endif
+
   #if HAS_LEDS_OFF_FLAG
     printerEventLEDs.onResumeAfterWait();
   #endif