Support ::slotted pseudo selector

[xzyfer]

See sass/sass#2266
Spec https://drafts.csswg.org/css-scoping-1/#slotted-pseudo

[mgreter]

This is related to #2055 and IMHO already supported (at least for the generic cases, not sure if ruby sass has specific superselector rules for this as for :not or :has). In general peudo selector are not very specifically handled in libsass yet (specially in case of extending the wrapped selectors) => #2055.

Not sure it's worth keeping open here.
IMO a ticket in sass-spec would make more sense.
Then open specific tickets as special cases crop up.