Handle unknown at rules. Fix #51

[octref]

Known limitations:

Removed a test case from 7f07d5e which intended to address microsoft/vscode#2573 and microsoft/vscode#23862

Currently the parser rule is relaxed, so

// .something {
    @media (max-width: 760px) {
        > .test {
            color: blue;
        }
    }
// }

Will not give any errors, although > .test inside @media should really be used in a nested selector.

Did this because the current errors for this case is very wrong:

@aeschli I can log a new issue for giving correct error here, like Selectors that begin with a combinator should be used inside a nested RuleSet.