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}'.`);
    }
}
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.