function defaultOptions() {
    return {
        optimistic: false,
        nodejsScope: false,
        impliedStrict: false,
        sourceType: "script", // one of ['script', 'module', 'commonjs']
        ecmaVersion: 5,
        childVisitorKeys: null,
        fallback: "iteration"
    };
}
Set the default options
@returns {Object} options