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

src/stepper/test/__snapshots__/index.spec.js.snap

@@ -8,4 +8,6 @@ exports[`disabled stepper 1`] = `<div class="van-stepper"><button type="button"
 
 exports[`input-width prop 1`] = `<div class="van-stepper"><button type="button" class="van-stepper__minus van-stepper__minus--disabled"></button><input type="text" role="spinbutton" inputmode="decimal" aria-valuemax="Infinity" aria-valuemin="1" aria-valuenow="1" class="van-stepper__input" style="width: 10rem;"><button type="button" class="van-stepper__plus"></button></div>`;
 
+exports[`placeholder prop 1`] = `<input type="text" role="spinbutton" inputmode="decimal" placeholder="foo" aria-valuemax="Infinity" aria-valuemin="1" aria-valuenow="1" class="van-stepper__input">`;
+
 exports[`show-plus & show-minus props 1`] = `<div class="van-stepper"><button type="button" class="van-stepper__minus van-stepper__minus--disabled" style="display: none;"></button><input type="text" role="spinbutton" inputmode="decimal" aria-valuemax="Infinity" aria-valuemin="1" aria-valuenow="1" class="van-stepper__input"><button type="button" class="van-stepper__plus" style="display: none;"></button></div>`;

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();
+});