Add support for a wider variety of at rules

Unknown at-rules stil get to be `RuleSet`s but block rules like document/supports/region-style/font-feature-values
are now supported via the AtRuleBlockList class.

Also, all at rules now implement the Sabberworm\CSS\Property\AtRule interface.

MediaQuery has been removed as its functionality is included fully in the AtRuleBlockList class.

Closes #53 as fixed

Author: sabberworm

lib/Sabberworm/CSS/CSSList/AtRuleBlockList.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace Sabberworm\CSS\CSSList;
+
+use Sabberworm\CSS\Property\AtRule;
+
+/**
+ * A RuleSet constructed by an unknown @-rule. @font-face rules are rendered into AtRule objects.
+ */
+class AtRuleBlockList extends CSSBlockList implements AtRule {
+
+	private $sType;
+	private $sArgs;
+
+	public function __construct($sType, $sArgs = '') {
+		parent::__construct();
+		$this->sType = $sType;
+		$this->sArgs = $sArgs;
+	}
+
+	public function atRuleName() {
+		return $this->sType;
+	}
+
+	public function atRuleArgs() {
+		return $this->sArgs;
+	}
+
+	public function __toString() {
+		$sResult = "@{$this->sType} {$this->sArgs}{";
+		$sResult .= parent::__toString();
+		$sResult .= '}';
+		return $sResult;
+	}
+
+}

lib/Sabberworm/CSS/CSSList/MediaQuery.php

@@ -0,0 +1,8 @@
+<?php
+
+namespace Sabberworm\CSS\Rule;
+
+interface AtRule {
+	public function atRuleName();
+	public function atRuleArgs();
+}