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/quantum.h

@@ -54,6 +54,10 @@
 #include <stddef.h>
 #include <stdlib.h>
 
+#ifdef DEFERRED_EXEC_ENABLE
+#    include "deferred_exec.h"
+#endif
+
 extern layer_state_t default_layer_state;
 
 #ifndef NO_ACTION_LAYER