JS: add Util::pluralize, also add tests for Util::capitalize

javascript/ql/src/semmle/javascript/Util.qll

@@ -11,3 +11,16 @@ bindingset[s]
 string capitalize(string s) {
   result = s.charAt(0).toUpperCase() + s.suffix(1)
 }
+
+ /**
+  * Gets the pluralization for `n` occurrences of `noun`.
+  *
+  * For example, the pluralization of `"function"` for `n = 2` is `"functions"`.
+  */
+bindingset[noun, n]
+string pluralize(string noun, int n) {
+  if n = 1 then
+    result = noun
+  else
+    result = noun + "s"
+}