Support 'preserved' variables and minor modes in `so-long-mode'

Support 'preserved' variables and minor modes in `so-long-mode'

The default values support preserving the state of `view-mode' when
switching to (and reverting from) `so-long-mode' (bug#45084).

* lisp/so-long.el (so-long-mode-preserved-variables)
(so-long-mode-preserved-minor-modes): New user options.
(so-long-mode-maintain-preserved-variables)
(so-long-mode-maintain-preserved-minor-modes): New functions.
(so-long-remember-all, so-long-after-change-major-mode)
(so-long-mode-revert): Use them.

* etc/NEWS: Describe changes.

* test/lisp/so-long-tests/so-long-tests-helpers.el:
* test/lisp/so-long-tests/so-long-tests.el: Update tests.