feat(Form): support async validation

src/form/demo/ValidateRules.vue

@@ -1,13 +1,19 @@
 <template>
   <demo-block :title="$t('title')">
-    <van-form>
+    <van-form validate-first @sumbit="onSubmit" @failed="onFailed">
       <van-field
-        v-model="value"
+        v-model="phone"
+        name="phone"
         :label="$t('phone')"
-        :rules="rules"
+        :rules="rules.phone"
         :placeholder="$t('phone')"
-        @submit="onSubmit"
-        @failed="onFailed"
+      />
+      <van-field
+        v-model="code"
+        name="code"
+        :label="$t('code')"
+        :rules="rules.code"
+        :placeholder="$t('code')"
       />
       <div style="margin: 16px 16px 0;">
         <van-button type="info" round block>{{ $t('submit') }}</van-button>
@@ -20,35 +26,57 @@
 export default {
   i18n: {
     'zh-CN': {
+      code: '验证码',
       phone: '手机号',
       title: '校验规则',
       submit: '提交',
       required: '请输入手机号',
-      incorrectFormat: '手机号格式错误',
+      validating: '验证中...',
+      incorrectCode: '验证码错误',
+      incorrectPhone: '手机号格式错误',
     },
     'en-US': {
+      code: 'Code',
       phone: 'Phone',
       title: 'Validate Rules',
       submit: 'Submit',
-      required: 'Phone is required',
-      incorrectFormat: 'Incorrect format',
+      validating: 'Validating...',
+      incorrectCode: 'Incorrect code',
+      incorrectPhone: 'Incorrect phone',
     },
   },
 
   data() {
     return {
-      value: '',
+      code: '',
+      phone: '',
     };
   },
 
   created() {
-    this.rules = [
-      { required: true, message: this.$t('required') },
-      {
-        validator: val => /1\d{10}/.test(val),
-        message: this.$t('incorrectFormat'),
-      },
-    ];
+    this.rules = {
+      phone: [
+        { required: true, message: this.$t('required') },
+        {
+          validator: val => /1\d{10}/.test(val),
+          message: this.$t('incorrectPhone'),
+        },
+      ],
+      code: [
+        {
+          validator: val =>
+            new Promise(resolve => {
+              this.$toast.loading(this.$t('validating'));
+
+              setTimeout(() => {
+                this.$toast.clear();
+                resolve(/\d{6}/.test(val));
+              }, 1000);
+            }),
+          message: this.$t('incorrectCode'),
+        },
+      ],
+    };
   },
 
   methods: {