[css-conditional] Make CSSSupportsRule expose whether its condition is met #4240

You currently can't tell directly from a CSSSupportsRule whether the condition is actually fulfilled or not. To do that you have to use CSS.supports(cssSupportsRule.conditionText).

Therefore I suggest to add a getter to CSSSupportsRule (or rather to CSSConditionRule which is overwritten by the derived interfaces) that returns whether the condition is met or not, e.g supported or fulfilled.

Sebastian