Support xs:all element

[Tails]

Thank you for this awesome repo! Don't hesitate to host on crates.io.

I am trying to parse a schema that has an xs:all element, which seems to be unsupported still.

thread 'main' panicked at 'internal error: entered unreachable code: Unsupported node:
 Element { tag_name: {http://www.w3.org/2001/XMLSchema}all, attributes: [], namespaces: [Namespace { name: Some("xs"), uri: "http://www.w3.org/2001/XMLSchema" }, Namespace { name: None, uri: "http://www.customer.com/Risks" }] }
parent = Some(Element { tag_name: {http://www.w3.org/2001/XMLSchema}complexType, attributes: [Attribute { name: name, value: "Risk" }], namespaces: [Namespace { name: Some("xs"), uri: "http://www.w3.org/2001/XMLSchema" }, Namespace { name: None, uri: "http://www.customer.com/Risks" }] })
', xsd-parser/src/parser/node_parser.rs:43:14

 <xs:complexType name="Risk">
        <xs:all>
           ...
        </xs:all>
</xs:complexType>