Update dependency xmldom

Switching from package `xmldom` to `@xmldom/xmldom`, which resolves the security issue present in latest xmldom version 0.6.0:
GHSA-5fg8-2547-mr8q

The reason is that the maintainers were forced to switch to a scoped package since 0.7.0:
 xmldom/xmldom#271

- I only changed all occurences of `xmldom`, I didn't execute `npm install` or any other scripts.
- Since there is no package-lock.json, I have no idea what version of xmldom people were actually using (depends on the time of running `npm install`), so I'm just assuming everybody was using 0.6.0
- fixes #41

I'm one of the xmldom maintainers. Don't hesitate to ask me questions about it.

lib/sepa.js

@@ -759,7 +759,7 @@
     if (typeof document !== 'undefined' && typeof document.implementation !== 'undefined') {
       return document.implementation.createDocument(nsURI, qname);
     } else {
-      var DOMImplementation = require('xmldom').DOMImplementation;
+      var DOMImplementation = require('@xmldom/xmldom').DOMImplementation;
       return new DOMImplementation().createDocument(nsURI, qname);
     }
   }
@@ -774,7 +774,7 @@
   function serializeToString(doc) {
     var s;
     if (typeof window === 'undefined') {
-      var XMLSerializer = require('xmldom').XMLSerializer;
+      var XMLSerializer = require('@xmldom/xmldom').XMLSerializer;
       s = new XMLSerializer();
     } else {
       s = new window.XMLSerializer();