XML: added XMLNode API to modify XML documents.

XML: added XMLNode API to modify XML documents.

    - delete node.$attr$a is a shorthand syntax for
        node.removeAttribute('a')

    - node.removeAllAttributes() removes all attributes of the node.

    - node.removeAttribute(attr_name) removes attribute named
        attr_name.

    - node.setAttribute(attr_name, value || null) sets a value for an
    attr_name. When value is null attribute named attr_name is
    deleted.

    - node.$attr$a = 'xxx' is a shorthand syntax for
        node.setAttribute('a', 'xxx');

    - The methods and operations below make copy-on-write
        changes to the original XML parsed document.

        For example:
        var doc = xml.parse(<r><a><b/></a></r>);
        var b = doc.$root.a.b;
        doc.$root.removeAllChildren();

        console.log((new TextDecoder()).decode(xml.c14n(doc))); /* <r></r> */
        console.log(b); /* XMLNode {$name:'b'} */
        console.log(b.$parent); /* XMLNode {$name:'a',$tags:[XMLNode
                                  {$name:'b'}] */

        "b" is valid after removeAllChildren() call, but is not a part
        of the document tree anymore.

    - node.addChild(nd) adds XMLNode as a child to node.
      nd recursively copied before adding to the node.

    - node.removeChildren(tag_name?) removes all the children tags
        named tag_name. If tag_name is absent all children tags are
        removed.

    - node.removeText() removes the node's text value.

    - node.setText(string || null) sets a text value for the node.
        When value is null the node's text is deleted.

    - node.$tags = [node1, node2, ..] is a shorthand syntax for
        node.removeChildren();
        node.addChild(node1);
        node.addChild(node2)

    - node.$text = 'xxx' is a shorthand syntax for
        node.setText('xxx');

    In addition the following method were added:
    - xml.serialize() is the same as xml.c14n()

    - xml.serializeToString() is the same as xml.c14n()
        except it returns the result as string.

    Example:
        const xml = require("xml");
        let data = `<note><to b="bar" a="foo">Tove</to><from>Jani</from></note>`;
        let doc = xml.parse(data);

        doc.$root.to.$attr$b = 'bar2';
        doc.$root.to.setAttribute('c', 'baz');
        delete doc.$root.to.$attr$a;

        console.log(xml.serializeToString(doc.$root.to)) /* '<to b="bar2" c="baz">Tove</to>' */

        doc.$root.to.removeAllAttributes();
        doc.$root.from.$text = 'Jani2';

        console.log(xml.serializeToString(doc)) /* '<note><to>Tove</to><from>Jani2</from></note>' */

        doc.$root.to.$tags = [xml.parse(`<a/>`), xml.parse(`<b/>`)];
        doc.$root.to.addChild(xml.parse(`<a/>`));

        console.log(xml.serializeToString(doc.$root.to)) /* '<to><a></a><b></b><a></a></to>' */

        doc.$root.to.removeChildren('a');

        console.log(xml.serializeToString(doc.$root.to)) /* '<to><b></b></to>' */