feat(Stepper): add placeholder prop (#6519)

src/stepper/test/index.spec.js

@@ -378,3 +378,12 @@ test('change integer', async () => {
 
   expect(wrapper.emitted('input')[0][0]).toEqual(1);
 });
+
+test('placeholder prop', () => {
+  const wrapper = mount(Stepper, {
+    propsData: {
+      placeholder: 'foo',
+    },
+  });
+  expect(wrapper.find('.van-stepper__input')).toMatchSnapshot();
+});