feat(backtop): add backtop (#71)

src/layout/components/appMain.vue

@@ -16,6 +16,7 @@ const transition = computed(() => {
 <template>
   <section class="app-main">
     <el-scrollbar>
+      <el-backtop target=".app-main .el-scrollbar__wrap"></el-backtop>
       <router-view>
         <template #default="{ Component, route }">
           <transition

src/plugins/element-plus/index.ts

@@ -33,7 +33,8 @@ import {
   ElRadioButton,
   ElRadioGroup,
   ElDescriptions,
-  ElDescriptionsItem
+  ElDescriptionsItem,
+  ElBacktop
 } from "element-plus";
 
 const components = [
@@ -69,7 +70,8 @@ const components = [
   ElRadioButton,
   ElRadioGroup,
   ElDescriptions,
-  ElDescriptionsItem
+  ElDescriptionsItem,
+  ElBacktop
 ];
 
 const plugins = [ElLoading];