code: function checkLanguageOption(options, optionName) {
      const language = options[optionName];
      if (language !== undefined && loadPrismLang(language) === undefined) {
          throw new Error(`Bad option ${optionName}: There is no Prism language '${language}'.`);
      }
  }
docs: Checks whether an option represents a valid Prism language
  
  @param {MarkdownItPrismOptions} options
  The options that have been used to initialise the plugin.
  @param optionName
  The key of the option insides {@code options} that shall be checked.
  @throws {Error} If the option is not set to a valid Prism language.