feat(Button): add loading slot (#6966)

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

@@ -7,6 +7,12 @@ exports[`icon-prefix prop 1`] = `
 </button>
 `;
 
+exports[`loading slot 1`] = `
+<button class="van-button van-button--default van-button--normal van-button--loading">
+  <div class="van-button__content">Custom Loading</div>
+</button>
+`;
+
 exports[`loading-size prop 1`] = `
 <button class="van-button van-button--default van-button--normal van-button--loading">
   <div class="van-button__content">

src/button/test/index.spec.js

@@ -93,3 +93,16 @@ test('icon-prefix prop', () => {
 
   expect(wrapper).toMatchSnapshot();
 });
+
+test('loading slot', () => {
+  const wrapper = mount(Button, {
+    propsData: {
+      loading: true,
+    },
+    scopedSlots: {
+      loading: () => 'Custom Loading',
+    },
+  });
+
+  expect(wrapper).toMatchSnapshot();
+});