[new feature] DropdownItem: add disabled prop

packages/dropdown-menu/test/__snapshots__/demo.spec.js.snap

@@ -26,5 +26,17 @@ exports[`renders demo correctly 1`] = `
       </div>
     </div>
   </div>
+  <div>
+    <div class="van-dropdown-menu van-hairline--top-bottom">
+      <div class="van-dropdown-menu__item van-dropdown-menu__item--disabled"><span class="van-dropdown-menu__title">全部商品</span></div>
+      <div class="van-dropdown-menu__item van-dropdown-menu__item--disabled"><span class="van-dropdown-menu__title">默认排序</span></div>
+      <div class="van-dropdown-item" style="top: 0px; z-index: 10; display: none;">
+        <!---->
+      </div>
+      <div class="van-dropdown-item" style="top: 0px; z-index: 10; display: none;">
+        <!---->
+      </div>
+    </div>
+  </div>
 </div>
 `;

packages/dropdown-menu/test/__snapshots__/index.spec.js.snap

@@ -45,6 +45,15 @@ exports[`didn\`t find matched option 1`] = `
 </div>
 `;
 
+exports[`disable dropdown item 1`] = `
+<div class="van-dropdown-menu van-hairline--top-bottom">
+  <div class="van-dropdown-menu__item van-dropdown-menu__item--disabled"><span class="van-dropdown-menu__title">A</span></div>
+  <div class="van-dropdown-item" style="top: 0px; z-index: 10; display: none;">
+    <!---->
+  </div>
+</div>
+`;
+
 exports[`show dropdown item 1`] = `
 <div class="van-dropdown-menu van-hairline--top-bottom">
   <div class="van-dropdown-menu__item"><span class="van-dropdown-menu__title van-dropdown-menu__title--active" style="color: rgb(25, 137, 250);">A</span></div>

packages/dropdown-menu/test/index.spec.js

@@ -99,6 +99,32 @@ test('destroy one item', async () => {
   expect(wrapper).toMatchSnapshot();
 });
 
+test('disable dropdown item', async () => {
+  const wrapper = mount({
+    template: `
+      <dropdown-menu>
+        <dropdown-item disabled v-model="value" :options="options" />
+      </dropdown-menu>
+    `,
+    components: {
+      DropdownItem,
+      DropdownMenu
+    },
+    data() {
+      return {
+        value: 0,
+        options: [
+          { text: 'A', value: 0 },
+          { text: 'B', value: 1 }
+        ]
+      };
+    }
+  });
+
+  const title = wrapper.find('.van-dropdown-menu__title');
+  title.trigger('click');
+  expect(wrapper).toMatchSnapshot();
+});
 
 test('change event', async () => {
   const onChange = jest.fn();