add `href` in transformAssetUrls default option for `image` element 

like this
```javascript
const defaultOptions: AssetURLOptions = {
  video: ['src', 'poster'],
  source: 'src',
  img: 'src',
  image: ['xlink:href', 'href']
}
```
`xlink:href` was deprecated since SVG 2 ([MDN](https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/xlink:href)), also keep `xlink:href` for compatibility.