root scope: @this and @root

### hss file

```
@this {
//this should be matched by the root element
property: keyword;
}

@root {
//and this as well obviously
property: keyword;
}
```

Starting its hss file with `@root {` might become the norm since  `* {` can be confusing in case you embed another xml document. Even if it's sanboxed, users could understandably think it's matching its siblings as well.
