New minor mode elide-head-mode

New minor mode elide-head-mode

* lisp/elide-head.el (elide-head-mode): New minor mode.  (Bug#52980)
(elide-head--delete-overlay, elide-head--show)
(elide-head--hide): New functions factored out from ...
(elide-head, elide-head-show): ... here.  Make obsolete in favor
of elide-head-mode.
(elide-head-headers-to-hide): Doc fix.

* test/lisp/elide-head-tests.el (ert-x): Require.
(elide-head-tests-elide-head-mode)
(elide-head-tests-elide-head-mode/enable-disable)
(elide-head-tests-elide-head-mode/normal-mode)
(elide-head-tests-elide-head-mode/revert-buffer): New tests.
(elide-head--add-test): Update test to use elide-head-mode.
(elide-head-tests-elide-head)
(elide-head-tests-elide-head-with-prefix-arg)
(elide-head-tests-show): Make obsolete.