parse static method
Implementation
static CSSURL? parse(String? url) {
if (url == null) return null;
url = url.trim();
if (url.isEmpty) return null;
var match = pattern.firstMatch(url);
if (match == null) return null;
var uri = match.group(1) ?? match.group(2) ?? match.group(3);
return CSSURL(uri);
}