Add jsxmlserializer

[juancarlospaco]

• Added jsxmlserializer module for XMLSerializer for the JavaScript target.
• https://developer.mozilla.org/en-US/docs/Web/API/XMLSerializer
• Serialize DOM to XML very fast. Tiny diff.

let cerealizer: XMLSerializer = newXMLSerializer()
echo cerealizer.serializeToString(node = document)

[metagn]

Can you make a case for why this should be in the standard library? I think anything in "Web APIs" on MDN should be in nimble/fusion.

[Araq]

Please add it to fusion instead.