feat: input新增extra插槽

#1639

Author: Boomkaa

src/input/_example/suffix/index.wxml

@@ -9,4 +9,8 @@
   <t-button slot="suffix" theme="primary" size="extra-small"> 操作按钮 </t-button>
 </t-input>
 
+<t-input label="标签文字" placeholder="请输入文字" tips="最多十个字">
+  <t-button slot="extra" theme="primary" maxcharacter="{{10}}" size="extra-small"> 操作按钮 </t-button>
+</t-input>
+
 <t-input label="标签文字" placeholder="请输入文字" suffixIcon="{{ { name: 'user-avatar', ariaLabel: '通讯录' } }}" />

src/input/input.wxml

@@ -98,4 +98,6 @@
     </view>
     <slot name="tips" />
   </view>
+
+  <slot name="extra" />
 </view>