Add 'forward-sexp-default-function' to be used by 'treesit-forward-sexp'

Add 'forward-sexp-default-function' to be used by 'treesit-forward-sexp'

* lisp/emacs-lisp/lisp.el (forward-sexp-default-function):
New function with body from 'forward-sexp' (bug#68993).
(forward-sexp-function): Change the default value from nil to
'forward-sexp-default-function'.
(forward-sexp): Use either 'forward-sexp-function' or
'forward-sexp-default-function'.

* lisp/treesit.el (treesit-forward-sexp): In nodes of type 'text'
fall back to 'forward-sexp-default-function'.  Improve docstring.

* doc/lispref/positions.texi (List Motion): Fix pxref.