Defer triggering `so-long' until the buffer is displayed

Defer triggering `so-long' until the buffer is displayed

* lisp/so-long.el (so-long-invisible-buffer-function): New user option.
(so-long--set-auto-mode): Use so-long-invisible-buffer-function.
(so-long-deferred): New function/value for so-long-invisible-buffer-function.
(so-long, so-long--disable): Support for so-long-deferred.

* test/lisp/so-long-tests/autoload-longlines-mode-tests.el
* test/lisp/so-long-tests/autoload-major-mode-tests.el
* test/lisp/so-long-tests/autoload-minor-mode-tests.el
* test/lisp/so-long-tests/so-long-tests.el:
Support for so-long-deferred.

Pre-existing tests have been updated to ensure the buffer is already
displayed in cases where a call to `normal-mode' is the (potential)
trigger for `so-long'.