zjsquery: Add data support.

Before this we just noop'ed it, since at one time
we were trying to deprecate this is in favor
of attr calls.

frontend_tests/node_tests/zjquery.js

@@ -50,6 +50,11 @@ run_test('basics', () => {
 
     widget.attr('data-employee-id', 42);
     assert.equal(widget.attr('data-employee-id'), 42);
+    assert.equal(widget.data('employee-id'), 42);
+
+    widget.data('department-id', 77);
+    assert.equal(widget.attr('data-department-id'), 77);
+    assert.equal(widget.data('department-id'), 77);
 
     widget.html('<b>hello</b>');
     assert.equal(widget.html(), '<b>hello</b>');

frontend_tests/zjsunit/zjquery.js

@@ -177,7 +177,17 @@ exports.make_new_elem = function (selector, opts) {
             }
             return elem.parent().closest(selector);
         },
-        data: noop,
+        data: function (name, val) {
+            if (val === undefined) {
+                const data_val = attrs.get('data-' + name);
+                if (data_val === undefined) {
+                    return;
+                }
+                return JSON.parse(data_val);
+            }
+            attrs.set('data-' + name, val);
+            return self;
+        },
         delay: function () {
             return self;
         },