fixes parser error for css ::host and ::sloted pseudo-elements

[haidubogdan]

Fixes parser error display for :host and :slotted pseudo-elements.
The solution is a bit of compromise and necessity as these pseudo elements use a simple compound selector compared to :where, :is, :has and the host pseudo class can work without a selector and I needed to avoid the one rule node fail in antrl3.
The Css3.g antlr 3 rules would benefit from a simplification in the future :) .

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[matthiasblaesing]

Change looks sane to me. Thank you.