* lisp/subr.el (add-hook): Turn `append` into `depth` (bug#35508)

* lisp/subr.el (add-hook): Turn `append` into `depth` (bug#35508)

Make it possible to control the relative ordering of functions on hooks by
specifying `depth` in the same was as was possible with `add-function`.

* lisp/electric.el (electric--sort-post-self-insertion-hook):
Delete function.
(electric-indent-mode, electric-layout-mode, electric-quote-mode):
* lisp/elec-pair.el (electric-pair-mode): Use new `depth` arg instead of
electric--sort-post-self-insertion-hook.

* lisp/emacs-lisp/syntax.el (syntax-propertize, syntax-ppss):
Use new `depth` arg to make sure noone accidentally gets added
after syntax-ppss-flush-cache.

* doc/lispref/modes.texi (Setting Hooks): Document new `depth` arg.

* test/lisp/subr-tests.el (subr-tests-add-hook-depth): New test.