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>

builddefs/generic_features.mk

@@ -19,6 +19,7 @@ GRAVE_ESC_ENABLE ?= yes
 GENERIC_FEATURES = \
     COMBO \
     COMMAND \
+    DEFERRED_EXEC \
     DIGITIZER \
     DIP_SWITCH \
     DYNAMIC_KEYMAP \