Add addClone method to XmlElement

[afn]

Add an addClone function to XmlElement to allow adding cloned children from one document to another.

Example:

const { XmlDocument } = await import ('libxml2-wasm');
const doc1 = XmlDocument.fromString('<foo><bar>hello</bar></foo>');
const doc2 = XmlDocument.fromString('<root><baz>Ahoy matey!</baz></root>');
const foo = doc1.get('/foo');
const baz = doc2.get('//baz');
foo.addClone(baz);
console.log(doc1.toString());

outputs:

<?xml version="1.0" encoding="utf-8"?>
<foo>
  <bar>hello</bar>
  <baz>Ahoy matey!</baz>
</foo>