[new feature] Toast: add iconPrefix option (#3872)

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

@@ -12,6 +12,13 @@ exports[`icon prop 1`] = `
 </div>
 `;
 
+exports[`icon-prefix prop 1`] = `
+<div class="van-toast van-toast--middle" style="z-index: 2005;" name="van-fade"><i class="my-icon my-icon-star-o van-toast__icon">
+    <!----></i>
+  <div class="van-toast__text">Message</div>
+</div>
+`;
+
 exports[`show html toast 1`] = `
 <div class="van-toast van-toast--middle van-toast--text" style="z-index: 2003;" name="van-fade">
   <div class="van-toast__text">

src/toast/test/index.spec.js

@@ -60,6 +60,17 @@ test('icon prop', async () => {
   expect(toast.$el.outerHTML).toMatchSnapshot();
 });
 
+test('icon-prefix prop', async () => {
+  const toast = Toast({
+    message: 'Message',
+    icon: 'star-o',
+    iconPrefix: 'my-icon'
+  });
+
+  await later();
+  expect(toast.$el.outerHTML).toMatchSnapshot();
+});
+
 test('clear toast', () => {
   const toast1 = Toast();
   expect(toast1.value).toBeTruthy();