LibWeb: Add and implement CSSPropertyRule IDL and bindings

Author: https://github.com/a-ungurianu 🔰
Commit: 50d64b0fb7
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1777
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/AtkinsSJ ✅

Userland/Libraries/LibWeb/CSS/CSSPropertyRule.h

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+/*
+ * Copyright (c) 2024, Alex Ungurianu <alex@ungurianu.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/FlyString.h>
+#include <AK/NonnullRefPtr.h>
+#include <AK/Optional.h>
+#include <AK/String.h>
+#include <LibWeb/CSS/CSSRule.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::CSS {
+
+// https://drafts.css-houdini.org/css-properties-values-api/#the-css-property-rule-interface
+class CSSPropertyRule final : public CSSRule {
+    WEB_PLATFORM_OBJECT(CSSPropertyRule, CSSRule);
+    JS_DECLARE_ALLOCATOR(CSSPropertyRule);
+
+public:
+    static JS::NonnullGCPtr<CSSPropertyRule> create(JS::Realm&, FlyString name, FlyString syntax, bool inherits, Optional<String> initial_value);
+
+    virtual ~CSSPropertyRule() = default;
+
+    virtual Type type() const override { return Type::Property; }
+    FlyString const& name() const { return m_name; }
+    FlyString const& syntax() const { return m_syntax; }
+    bool inherits() const { return m_inherits; }
+    Optional<String> initial_value() const { return m_initial_value; }
+
+private:
+    CSSPropertyRule(JS::Realm&, FlyString name, FlyString syntax, bool inherits, Optional<String> initial_value);
+
+    virtual void initialize(JS::Realm&) override;
+    virtual String serialized() const override;
+
+    FlyString m_name;
+    FlyString m_syntax;
+    bool m_inherits;
+    // FIXME: This should hold an actual CSS value, matching the syntax
+    Optional<String> m_initial_value;
+};
+
+template<>
+inline bool CSSRule::fast_is<CSSPropertyRule>() const { return type() == CSSRule::Type::Property; }
+
+}