Backtop: Add backtop component (#15541)

types/backtop.d.ts

@@ -0,0 +1,16 @@
+import { ElementUIComponent } from './component'
+
+/** Backtop Component */
+export declare class ElBacktop extends ElementUIComponent {
+  /** Backtop target */
+  target: string
+  
+  /** Backtop visibility height */
+  visibilityHeight: string | number
+
+  /** Backtop right position */
+  right: string | number
+
+  /** Backtop bottom position */
+  bottom: string | number
+}

types/element-ui.d.ts

@@ -74,6 +74,7 @@ import { ElDivider } from './divider'
 import { ElIcon } from './icon'
 import { ElCalendar } from './calendar'
 import { ElImage } from './image'
+import { ElBacktop } from './backtop'
 import { ElInfiniteScroll } from './infiniteScroll'
 
 export interface InstallationOptions {
@@ -322,5 +323,9 @@ export class Icon extends ElIcon {}
 /** Calendar Component */
 export class Calendar extends ElCalendar {}
 
+/** Backtop Component */
+export class Backtop extends ElBacktop {}
+
 /** InfiniteScroll Component */
 export class InfiniteScroll extends ElInfiniteScroll {}
+