Split up and add tests for two page.el functions

Split up and add tests for two page.el functions

* lisp/textmodes/page.el (page--count-lines-page): New function
extracted from `count-lines-page'.
(count-lines-page): Extract main logic into `page--count-lines-page'.
(page--what-page); New function extracted from `what-page'.
(what-page): Extract main logic into `page--what-page'.

* test/lisp/textmodes/page-tests.el (page-tests-count-lines-page)
(page-tests-what-page): New tests for `page--count-lines-page' and
`page--what-page'.  (Bug#36009)