CSS checks: consider reporting unclosed parentheses

[tidoust]

The value definition syntax allows parentheses as literals that represent their corresponding tokens.

This allowance means the extra parenthesis in the definition of <hsl()> (see w3c/csswg-drafts#7362) could be parsed without triggering any error. In practice, there should never be an open parenthesis without a closing one at the same level. Our tests could make sure that this is the case.

(This seems low priority such typos should be rare and are relatively easy to spot)