[improvement] replace icons (#2304)

packages/field/demo/index.vue

@@ -16,7 +16,7 @@
           :label="$t('username')"
           :placeholder="$t('usernamePlaceholder')"
           clearable
-          icon="question"
+          icon="question-o"
           required
           @click-icon="$toast('question')"
         />

packages/field/en-US.md

@@ -28,7 +28,7 @@ Use `type` prop to custom diffrent type fields.
     required
     clearable
     label="Username"
-    icon="question"
+    icon="question-o"
     placeholder="Username"
     @click-icon="$toast('question')"
   />

packages/field/test/__snapshots__/demo.spec.js.snap

@@ -29,7 +29,7 @@ exports[`renders demo correctly 1`] = `
         <div class="van-cell__value">
           <div class="van-field__body"><input type="text" placeholder="请输入用户名" value="" class="van-field__control">
             <!---->
-            <div class="van-field__icon"><i class="van-icon van-icon-question" style="color:undefined;font-size:undefined;">
+            <div class="van-field__icon"><i class="van-icon van-icon-question-o" style="color:undefined;font-size:undefined;">
                 <!---->
                 <!----></i></div>
             <!---->

packages/field/zh-CN.md

@@ -30,7 +30,7 @@ Vue.use(Field);
     required
     clearable
     label="用户名"
-    icon="question"
+    icon="question-o"
     placeholder="请输入用户名"
     @click-icon="$toast('question')"
   />