[Improvement] use bem mixin (#934)

packages/toast/toast.vue

@@ -1,6 +1,6 @@
 <template>
   <transition name="van-fade">
-    <div class="van-toast" :class="[`van-toast--${displayStyle}`, `van-toast--${position}`]" v-show="value">
+    <div v-show="value" :class="b([displayStyle, position])">
       <!-- text only -->
       <div v-if="displayStyle === 'text'">{{ message }}</div>
       <div v-if="displayStyle === 'html'" v-html="message" />
@@ -8,8 +8,8 @@
       <!-- with icon -->
       <template v-if="displayStyle === 'default'">
         <loading v-if="type === 'loading'" color="white" />
-        <icon v-else class="van-toast__icon" :name="type" />
-        <div v-if="hasMessage" class="van-toast__text">{{ message }}</div>
+        <icon v-else :class="b('icon')" :name="type" />
+        <div v-if="hasMessage" :class="b('text')">{{ message }}</div>
       </template>
     </div>
   </transition>