message: add Callback type and callback property on FilePicker
diff: @@ -51,6 +51,11 @@ declare class FilePicker<P extends FilePicker.Options = FilePicker.Options> exte
  */
  extensions: string[];
  + /**
  + * A callback function to trigger once a file has been selected
  + */
  + callback: FilePicker.Callback | undefined;
  +
  protected _loaded: boolean;
  /**
  @@ -327,6 +332,8 @@ declare namespace FilePicker {
  wildcard: string;
  }
  + type Callback = (path: string) => void;
  +
  interface Data {
  bucket: string | null;
  canGoBack: boolean;
  @@ -378,7 +385,7 @@ declare namespace FilePicker {
  button?: FilePicker['button'];
  - callback?: (path: string) => void;
  + callback?: Callback;
  /**
  * @defaultValue `'templates/apps/filepicker.html'`
  