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.vim b/src/testdir/test_gettext.vim
index a990121..ddfc402 100644
--- a/src/testdir/test_gettext.vim
+++ b/src/testdir/test_gettext.vim
@@ -13,6 +13,9 @@
   call assert_equal('xxxTESTxxx', gettext("xxxTESTxxx", "vim"))
   call assert_equal('xxxTESTxxx', gettext("xxxTESTxxx", "__PACKAGE__"))
   call assert_equal('ERROR: ', gettext("ERROR: ", "__PACKAGE__"))
+
+  call assert_equal('ERROR: ', ngettext("ERROR: ", "ERROR: ", 1, "__PACKAGE__"))
+  call assert_equal('ERRORS: ', ngettext("ERROR: ", "ERRORS: ", 2, "__PACKAGE__"))
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab