[w3c/webcomponents] Support for pseudo elements with ::slotted() selectors (#655)

This was discussed in Issue #331 but I don't think we came to a conclusion.

The CSS Scoping spec does not explicitly disallow pseudo elements in the "selector list" like the specs for  :not() and :matches() do. Neither does it currently explicitly allow pseudo elements after ::slotted.

Should we:

1. Not support other pseudo elements on ::slotted selectors
2. Support other pseudo elements inside ::slotted(): ::slotted(div::before)
3. Support other pseudo elements after ::slotted(): ::slotted(div)::before

@hayatoito @tabatkins

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