Add variable `fill-region-as-paragraph-function'

Add variable `fill-region-as-paragraph-function'

Add the variable `fill-region-as-paragraph-function' to provide
a way to override how functions like `fill-region' fill text.
* doc/lispref/text.texi (Filling): Document
`fill-region-as-paragraph-function' variable.
* doc/emacs/text.texi (Fill Commands): Reference
`fill-region-as-paragraph-function' variable.
* lisp/textmodes/fill.el (fill-region-as-paragraph-function):
Add variable.
(fill-region-as-paragraph): Convert into
`fill-region-as-paragraph-function' wrapper.
(fill-region-as-paragraph-default): Rename old
`fill-region-as-paragraph' function.
(fill-region-as-paragraph-semlf): Update calls to
`fill-region-as-paragraph-default'.
(fill-region): Add reference to `fill-region-as-paragraph-function'
in doc string.
* test/lisp/textmodes/fill-tests.el (fill-test-fill-region): Add
test case for the `fill-region' function.
* test/lisp/textmodes/fill-resources/fill-region.erts: Add test
data.  (Bug#78816)

(cherry picked from commit 99ec286d7803fa59f770013a2135f23918510103)