Re: [dom] matches, querySelector, etc. shouldn't throw on an unrecognized selector (#39)

I'm somewhat in favor of a Selector object, but if we're avoiding that, then having query() and queryAll() just not throw would be fine.

Re: a way to find if a selector is supported, I like foolip's idea - a `CSS.cleanSelector()` that returns the selector (with error clauses dropped) or null (if all comma-separated clauses were dropped).  Then maybe we can alter matches(), qS(), and qSA() to accept a `DOMString?` and just return false/an empty nodelist when passed a null, so you could write `el.matches(CSS.cleanSelector("foo"))` and it'll just work, without having to gate it behind an if statement.

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