DOCTYPE statement with external identifier getting trimmed to `<DOCTYPE html>`

It seems that `inline-css` is truncating the "external identifier" part of DOCTYPE.

Consider this example

```javascript
const inlineCss = require('inline-css')
inlineCss(
'',
{ url:'file://test.html' }
).then(console.debug)
//
```

Versions:
```
Node: v20.12.0
inline-css: 4.0.2
```

While browsers ignore most parts of DOCTYPE, this may be relevant for emails. I suspect this comes from Cheerio, but since `inline-css` puts more focus on emails, figured it may be more relevant to ask here. Is this the expected behavior?