feat(Button): add icon-position prop (#7174)

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

@@ -1,5 +1,12 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`icon-position prop 1`] = `
+<button class="van-button van-button--default van-button--normal">
+  <div class="van-button__content"><i class="van-icon van-icon-plus van-button__icon">
+      <!----></i></div>
+</button>
+`;
+
 exports[`icon-prefix prop 1`] = `
 <button class="van-button van-button--default van-button--normal">
   <div class="van-button__content"><i class="my-icon my-icon-success van-button__icon">

src/button/test/index.spec.js

@@ -11,6 +11,16 @@ test('loading-size prop', () => {
   expect(wrapper).toMatchSnapshot();
 });
 
+test('icon-position prop', () => {
+  const wrapper = mount(Button, {
+    propsData: {
+      icon: 'plus',
+      iconPosition: 'right',
+    },
+  });
+  expect(wrapper).toMatchSnapshot();
+});
+
 test('click event', () => {
   const onClick = jest.fn();
   const wrapper = mount(Button, {