Parse stylesheets with a tokenizer instead of a brace regex #536

Part of #520 (step 16).

CssParser::processCssString() splits the stylesheet with /(.*?)\{(.*?)\}/ (src/Css/CssParser.php:219), after url() values are protected with a %ZZ placeholder and @media blocks are unwrapped. A brace inside a string mangles its own rule. Whether the rule after it survives depends on whether the braces in the string happen to balance: q {quotes:"}" "{"} is mangled, and the next rule survives only because its two braces pair up.

Fix. Replace the regex with a tokenizer that handles strings containing {, } and ;, escapes, comments inside strings, and nested @media and @supports. It retires the %ZZ placeholder and the at-rule pass from #524. The selector tokenizer in #538 is separate and comes first.

Tests. Braces and semicolons inside strings and url(), escapes in selectors, comments inside strings, and nested at-rules.

Depends on #524. Related: #538.