Extend Outline mode with default visibility state

Extend Outline mode with default visibility state

* etc/NEWS: Announce support for default visibility state (bug#51809).

* lisp/outline.el (outline-mode, outline-minor-mode): Ensure default
visibility state is applied with outline-apply-default-state.
(outline-default-state, outline-default-rules)
(outline-default-long-line, outline-default-line-count): New defcustoms.
(outline-apply-default-state, outline-show-only-headings)
(outline--show-headings-up-to-level): New functions.