Implement XML data support

Implement XML data support

Example:

```
{{ with resources.Get "https://example.com/rss.xml" | transform.Unmarshal }}
    {{ range .channel.item }}
        <strong>{{ .title | plainify | htmlUnescape }}</strong><br />
        <p>{{ .description | plainify | htmlUnescape }}</p>
        {{ $link := .link | plainify | htmlUnescape }}
        <a href="{{ $link }}">{{ $link }}</a><br />
        <hr>
    {{ end }}
{{ end }}
```

Closes #4470