Refactor `DeclarationBlock::parse()`

[JakeQZ]

Following #1292, this method will have become quite bloated, and may exceed a desired cyclomatic complexity limit.

It could be broken down into helper methods, e.g. parseSelector and parseSelectorList.

[JakeQZ]

Looked at this again. I think it's as clean and clear as it can be.

The main work of the method is to parse the selector until it finds { or } not enclosed in quotes. It then calls on to parseRuleSet() to complete the job.

Pre-splitting at commas is not viable for the reasons outlined in #1398 (review).

That leaves the only refactoring possible being to remove one method call, add another method, and add a call to it. Which doesn't really help. Closing.

[JakeQZ]

Reopening. See #1398 (comment).

setSelectors() should include the parsing that parse() does, so as to allow selectors like a[title="this, and that"] to be passed.