Add comment about fallthrough in switch

Co-authored-by: Oliver Klee <github@oliverklee.de>

Author: JakeQZ

src/RuleSet/DeclarationBlock.php

@@ -47,6 +47,7 @@ public static function parse(ParserState $parserState, ?CSSList $list = null): ?
                 $nextCharacter = $parserState->peek();
                 switch ($nextCharacter) {
                     case '\'':
+                        // The fallthrough is intentional.
                     case '"':
                         if (!isset($stringWrapperCharacter)) {
                             $stringWrapperCharacter = $nextCharacter;