Select: add clear event

examples/docs/en-US/select.md

@@ -665,6 +665,7 @@ Create and select new items that are not included in select options
 | change | triggers when the selected value changes | current selected value |
 | visible-change | triggers when the dropdown appears/disappears | true when it appears, and false otherwise |
 | remove-tag | triggers when a tag is removed in multiple mode | removed tag value |
+| clear | triggers when the clear icon is clicked in a clearable Select | — |
 
 ### Option Group Attributes
 | Attribute      | Description          | Type      | Accepted Values       | Default  |

examples/docs/zh-CN/select.md

@@ -660,6 +660,7 @@
 | change | 选中值发生变化时触发 | 目前的选中值 |
 | visible-change | 下拉框出现/隐藏时触发 | 出现则为 true，隐藏则为 false |
 | remove-tag | 多选模式下移除tag时触发 | 移除的tag值 |
+| clear | 可清空的单选模式下用户点击清空按钮时触发 | — |
 
 ### Option Group Attributes
 | 参数      | 说明          | 类型      | 可选值                           | 默认值  |

packages/select/src/select.vue

@@ -602,6 +602,7 @@
         event.stopPropagation();
         this.$emit('input', '');
         this.visible = false;
+        this.$emit('clear');
       },
 
       deleteTag(event, tag) {