Added variable addition example (#23047)

Added an example showing storing values in variables before adding them. Also added syntax highlighting to original code snippet.

guide/english/javascript/tutorials/add-two-numbers-with-javascript/index.md

@@ -3,4 +3,15 @@ title: Add Two Numbers with JavaScript
 ---
 JavaScript uses the `+` symbol for addition. It can also be used instead of `parseInt()` but that is beyond this.
 
-    var sum = 10 + 10;
+```javascript
+var sum = 10 + 10;
+```
+    
+Variables can be added in a similar fashion.
+
+```javascript
+var x = 10;
+var y = 10;
+var z = x + y;
+console.log(z);    // Will print 20 to the console
+```