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/runtime/doc/version9.txt b/runtime/doc/version9.txt
index 2e16e09..64f49a6 100644
--- a/runtime/doc/version9.txt
+++ b/runtime/doc/version9.txt
@@ -41660,6 +41660,7 @@
 			Channel or Blob variable
 |matchbufline()|	all the matches of a pattern in a buffer
 |matchstrlist()|	all the matches of a pattern in a List of strings
+|ngettext()|		lookup single/plural message translation
 |popup_setbuf()|	switch to a different buffer in a popup
 |str2blob()|		convert a List of strings into a blob