* lisp/delsel.el (delete-selection-helper): New function, extracted from

* lisp/delsel.el (delete-selection-helper): New function, extracted from
delete-selection-pre-hook.
(delete-selection-pre-hook): Use it.
(delete-selection-self-insert-function): New function.
(delete-selection-self-insert-hooks): New hook.
(self-insert-command, self-insert-iso): Use it.
* lisp/electric.el (electric-pair-syntax): New function, extracted from
electric-pair-post-self-insert-function.
(electric-pair-post-self-insert-function): Use it.
(electric-pair-delete-selection-self-insert-function): New function.
(electric-pair-mode): Require delsel and setup
delete-selection-self-insert-hooks.