Allow write-contents-functions to short-circuit buffer save

Allow write-contents-functions to short-circuit buffer save

Bug#28412

* lisp/files.el (basic-save-buffer): Re-arrange function so that
  write-contents-functions are run earlier. If they return non-nil,
  consider the buffer saved without requiring the buffer to be
  visiting a file.
  (save-some-buffers): This function should consider any buffer with a
  buffer-local value for write-contents-functions eligible for
  saving.
* test/lisp/files-tests.el (files-test-no-file-write-contents): New
  test.
* doc/lispref/files.texi (Saving Buffers): Mention in docs.
* etc/NEWS: And in NEWS.