Add Reverse String in Nit (#5191)

Author: liliann19

archive/n/nit/reverse_string.nit

@@ -0,0 +1,10 @@
+if args.length > 0 then
+    var word = args[0]
+    var reversed = ""
+    var i = word.length - 1
+    while i >= 0 do 
+        reversed += word.chars[i].to_s
+        i -= 1
+    end
+    printn reversed
+end