[Feature request] Support for forgiving selector parsing

[CatchABus]

Right now, parser will throw even in the case of :is() pseudo-class that contains an invalid selector (even if there are other valid selectors).

That pseudo-class apparent accepts a forgiving selector list: https://developer.mozilla.org/en-US/docs/Web/CSS/:is#forgiving_selector_parsing

For example, in the following case css will still apply to valid selectors

:is(ol, ul, ??$#$@#) {
  color: green;
}

[fb55]

This makes sense as a new option — PRs welcome!