LibWeb: Merge CSS Parser's QualifiedStyleRule and AtStyleRule

AtStyleRule being a subclass of QualifiedStyleRule was causing
problems when trying to distinguish between them. Combining them
and then distinguishing between them with a Type enum makes that
check simpler, and is in line with how similar checks are done
elsewhere in the parser.

Userland/Libraries/LibWeb/CSS/Parser/StyleRule.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2020-2021, the SerenityOS developers.
+ * Copyright (c) 2021, Sam Atkins <atkinssj@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefCounted.h>
+#include <AK/Vector.h>
+#include <LibWeb/CSS/Parser/StyleBlockRule.h>
+#include <LibWeb/CSS/Parser/StyleComponentValueRule.h>
+
+namespace Web::CSS {
+
+class StyleRule : public RefCounted<StyleRule> {
+    friend class Parser;
+
+public:
+    enum class Type {
+        At,
+        Qualified,
+    };
+
+    StyleRule(Type);
+    ~StyleRule();
+
+    Vector<StyleComponentValueRule> const& prelude() const { return m_prelude; }
+    StyleBlockRule const& block() const { return *m_block; }
+
+    String to_string() const;
+
+private:
+    Type const m_type;
+    String m_name; // At-rules only
+    Vector<StyleComponentValueRule> m_prelude;
+    RefPtr<StyleBlockRule> m_block;
+};
+
+}