feat: make box's contents can empty and add animated property (#271)

* feat: make box's contents can empty and add animated property

* Modify to original

Author: louis70109

lib/types.ts

@@ -1411,6 +1411,14 @@ export type FlexImage = {
    * Specify an [action object](https://developers.line.biz/en/reference/messaging-api/#action-objects).
    */
   action?: Action;
+  /**
+   * When this is `true`, an animated image (APNG) plays.
+   * You can specify a value of `true` up to three times in a single message.
+   * You can't send messages that exceed this limit.
+   * This is `false` by default.
+   * Animated images larger than 300 KB aren't played back.
+   */
+  animated?: Boolean;
 } & Offset;
 
 /**