How to use the @remirror/core-extensions.PlaceholderExtension function in @remirror/core-extensions

export const helpers = {
  getState: Cast(jest.fn()),
  portalContainer: new PortalContainer(),
  getTheme: () => defaultRemirrorThemeValue,
};

export const baseExtensions = [
  { extension: new DocExtension(), priority: 2 },
  { extension: new TextExtension(), priority: 2 },
  { extension: new ParagraphExtension(), priority: 2 },
];

export const extensions = [
  ...baseExtensions,
  { extension: new HistoryExtension(), priority: 2 },
  { extension: new PlaceholderExtension(), priority: 2 },
  { extension: new BoldExtension(), priority: 3 },
  { extension: new ItalicExtension(), priority: 3 },
  { extension: new UnderlineExtension(), priority: 3 },
  { extension: new BlockquoteExtension(), priority: 3 },
];

/**
 * Useful for testing
 */
export const ExtensionMap = {
  nodes: {
    blockquote: new BlockquoteExtension(),
    heading: new HeadingExtension(),
  },
  marks: {
    bold: new BoldExtension(),