chore: update type backtop (#2691)

Author: xrkffgg

types/back-top.d.ts

@@ -5,17 +5,19 @@
 import { AntdComponent } from './component';
 
 export declare class BackTop extends AntdComponent {
-  /**
-   * specifies the scrollable area dom node
-   * @default () => window
-   * @type Function
-   */
-  target: () => HTMLElement;
+  $props: {
+    /**
+     * specifies the scrollable area dom node
+     * @default () => window
+     * @type Function
+     */
+    target?: () => HTMLElement;
 
-  /**
-   * the BackTop button will not show until the scroll height reaches this value
-   * @default 400
-   * @type number
-   */
-  visibilityHeight: number;
+    /**
+     * the BackTop button will not show until the scroll height reaches this value
+     * @default 400
+     * @type number
+     */
+    visibilityHeight?: number;
+  };
 }