Add ES module CDN example

[daun]

• Add example loading swup as ES module from a CDN
• Use module/nomodule pattern
• Skypack? Unpkg with ?module param?
• Try in browser to make sure it's working

<!-- ES module code -->

<script type="module">
      import Swup from "https://unpkg.com/swup@3?module";
      const swup = new Swup();
</script>

<!-- Fallback non-module code -->

<script nomodule src="https://unpkg.com/swup@3"></script>
<script nomodule>
  const swup = new Swup();
</script>