feat: 新增 Attachments 组件，支持文件上传和拖放功能

Author: weilei

packages/element-x-ui/src/components/Attachments/index.js

@@ -0,0 +1,8 @@
+import ElXAttachments from './src/main.vue';
+
+/* istanbul ignore next */
+ElXAttachments.install = function (Vue) {
+  Vue.component(ElXAttachments.name, ElXAttachments);
+};
+
+export default ElXAttachments;