Support CSS `:host` selector

[raystubbs]

Is your feature request related to a problem? Please describe.
It seems like the CSS :host selector isn't supported yet. Would be great to have this.

Describe the solution you'd like
The :host selector, if found within stylesheets linked or inlined in a shadow DOM, or CSSStyleSheet objects adopted by a ShadowRoot (via ShadowRoot#adoptedStyleSheets); should target the shadow DOM's host element.

Describe alternatives you've considered
Nothing.

Additional context
Nothing.