MDL-55224 core: Import PHP-CSS-Parser into core

Part of MDL-55071

lib/php-css-parser/Property/AtRule.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace Sabberworm\CSS\Property;
+
+use Sabberworm\CSS\Renderable;
+use Sabberworm\CSS\Comment\Commentable;
+
+interface AtRule extends Renderable, Commentable {
+	const BLOCK_RULES = 'media/document/supports/region-style/font-feature-values';
+	// Since there are more set rules than block rules, we’re whitelisting the block rules and have anything else be treated as a set rule.
+	const SET_RULES = 'font-face/counter-style/page/swash/styleset/annotation'; //…and more font-specific ones (to be used inside font-feature-values)
+	
+	public function atRuleName();
+	public function atRuleArgs();
+}