message: [bugfix] ImagePreview: fix ts definition
diff: import { VanPopupMixin } from './mixins/popup';
  +export type ImagePreviewOptions = string[] | {
  + images: string[];
  + startPosition?: number;
  + onClose?: () => any;
  +};
  +
  export class VanImagePreview extends VanPopupMixin {
  images: string[];
  startPosition: number;
  }
  export interface ImagePreview {
  - (images: string[], startPosition?: number): VanImagePreview;
  + (options: ImagePreviewOptions, startPosition?: number): VanImagePreview;
  install(): void;
  }
  