feat: support foreign elements

[fcrozatier]

svg and math elements have different parsing rules: self-closing elements, case-sensitive tage names etc

const markup =
    `<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100">
<!-- A self-closing tag -->
<circle cx="50" cy="50" r="40" stroke="green" fill="yellow"/>

<!-- Another self-closing tag but without the slash (valid in HTML, not in XML) -->
<rect x="10" y="10" width="30" height="30">

  <!-- Embedded foreign content  -->
  <script type="application/ecmascript">
      console.log("Inside SVG &lt;script&gt;");
  </script>

  <!-- Nested elements with same tag name -->
  <g>
    <g>
      <text x="10" y="90">Nested <tspan font-weight="bold">text</tspan> inside</text>
    </g>
  </g>

  <!-- Namespaced element -->
  <animateTransform attributeName="transform" type="rotate" from="0 50 50" to="360 50 50" dur="10s" repeatCount="indefinite"/>
</rect>
</svg>`;

const svg = element.parseOrThrow(markup);

assertEquals(serializeNode(svg), markup.replaceAll("/>", ">"));