:host and ::slotted parsing sort of wrong

This is a follow up issue to the recently merged PR. 
I figured that 
```
(<S>)*
( simpleSelectorSequence() )? 
```
is not correct. 
It should rather be
```
(<S>)*
( simpleSelectorSequence() (<S>)* )* 
```
 see: https://developer.mozilla.org/en-US/docs/Web/CSS/::slotted

Although i'm not quite sure about the trailing (<S>)* here. WDYT?

I'll file a PR soonish