LibWeb: Rename StyleDeclarationRule -> Declaration

This is the term used in the CSS specs.

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

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2020-2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/String.h>
+#include <AK/Vector.h>
+#include <LibWeb/CSS/CSSStyleDeclaration.h>
+#include <LibWeb/CSS/Parser/ComponentValue.h>
+
+namespace Web::CSS {
+
+class Declaration {
+    friend class Parser;
+
+public:
+    Declaration();
+    ~Declaration();
+
+    String to_string() const;
+
+private:
+    String m_name;
+    Vector<ComponentValue> m_values;
+    Important m_important { Important::No };
+};
+
+}