zjquery: Support $.fn.foo mechanism.

We can now extend zjquery using the $.fn mechanism.  This isn't
necessarily recommended for test code (since you can just stub
individual objects directly), but some of our real code does this.

frontend_tests/node_tests/zjquery.js

@@ -174,3 +174,26 @@ set_global('$', global.make_zjquery());
     obj2.addClass('.striped');
     assert(obj2.hasClass('.striped'));
 }());
+
+(function test_extensions() {
+    // You can extend $.fn so that all subsequent objects
+    // we create get a new function.
+
+    $.fn.area = function () {
+        return this.width() * this.height();
+    };
+
+    // Before we use area, though, let's illustrate that
+    // the predominant Zulip testing style is to stub objects
+    // using direct syntax:
+
+    var rect = $.create('rectangle');
+    rect.width = () => { return 5; };
+    rect.height = () => { return 7; };
+
+    assert.equal(rect.width(), 5);
+    assert.equal(rect.height(), 7);
+
+    // But we also have area available from general extension.
+    assert.equal(rect.area(), 35);
+}());

frontend_tests/zjsunit/zjquery.js

@@ -6,6 +6,15 @@ exports.make_zjquery = function () {
 
     var elems = {};
 
+    // Our fn structure helps us simulate extending jQuery.
+    var fn = {};
+
+    function add_extensions(obj) {
+        _.each(fn, (v, k) => {
+            obj[k] = v;
+        });
+    }
+
     function new_elem(selector) {
         var html = 'never-been-set';
         var text = 'never-been-set';
@@ -281,6 +290,8 @@ exports.make_zjquery = function () {
 
         self[0] = 'you-must-set-the-child-yourself';
 
+        add_extensions(self);
+
         return self;
     }
 
@@ -374,6 +385,8 @@ exports.make_zjquery = function () {
         return _.extend(content, container);
     };
 
+    zjquery.fn = fn;
+
     return zjquery;
 };