Add support for deferred executors. (#14859)

* Add support for deferred executors.

* More docs.

* Include from quantum.h

* Cleanup.

* Parameter checks

* Comments.

* qmk format-c

* I accidentally a few words.

* API name change.

* Apply suggestions from code review

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

* Review comments.

* qmk format-c

* Review comments.

Co-authored-by: Sergey Vlasov <sigprof@gmail.com>

quantum/main.c

@@ -43,6 +43,10 @@ void protocol_task(void) {
     protocol_post_task();
 }
 
+#ifdef DEFERRED_EXEC_ENABLE
+void deferred_exec_task(void);
+#endif  // DEFERRED_EXEC_ENABLE
+
 /** \brief Main
  *
  * FIXME: Needs doc
@@ -58,6 +62,12 @@ int main(void) {
     /* Main loop */
     while (true) {
         protocol_task();
+
+#ifdef DEFERRED_EXEC_ENABLE
+        // Run deferred executions
+        deferred_exec_task();
+#endif  // DEFERRED_EXEC_ENABLE
+
         housekeeping_task();
     }
 }