patch 9.1.1064: not possible to use plural forms with gettext()

Problem:  not possible to use plural forms with gettext()
Solution: implement ngettext() Vim script function (Christ van Willegen)

closes: #16561

Signed-off-by: Christ van Willegen <cvwillegen@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_gettext_cp1251.vim b/src/testdir/test_gettext_cp1251.vim
index 69d2bbf..9c30e09 100644
--- a/src/testdir/test_gettext_cp1251.vim
+++ b/src/testdir/test_gettext_cp1251.vim
@@ -14,7 +14,7 @@
 
     try
       language messages ru_RU
-      call assert_equal('ÎØÈÁÊÀ: ', gettext("ERROR: ", "__PACKAGE__"))
+      call assert_equal('ÎØÈÁÊÀ: for __PACKAGE__', gettext("ERROR: ", "__PACKAGE__"))
     catch /^Vim\%((\a\+)\)\=:E197:/
       throw "Skipped: not possible to set locale to ru (missing?)"
     endtry