Make the options argument actually optional

Fixes #23

Unfortunately this bug snuck through because the unit tests themselves
aren't yet written in TypeScript, so the test for `parseXml()` without
an `options` argument happily passed despite the type definition being
incorrect. 🤦‍♂️

src/index.ts

@@ -25,6 +25,6 @@ export type { ParserOptions } from './lib/Parser.js';
  * @param xml XML string to parse.
  * @param options Parser options.
  */
-export function parseXml(xml: string, options: ParserOptions) {
+export function parseXml(xml: string, options?: ParserOptions) {
   return (new Parser(xml, options)).document;
 }