LibWeb: Add SVGSVGElement.viewBox attribute

This attribute has some compatbility issues...
- The spec says it should be an SVGAnimatedRect which contains
  a DOMRect and a DOMReadOnlyRect.
- Blink gives you an SVGAnimatedRect with 2x SVGRect
- Gecko gives you an SVGAnimatedRect with 2x SVGRect? (nullable)

I ended up with something similar to Gecko, an SVGAnimatedRect
with 2x DOMRect? (nullable)

With this fixed, we can now load https://polar.sh/ :^)

Userland/Libraries/LibWeb/SVG/SVGAnimatedRect.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2024, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/Bindings/PlatformObject.h>
+#include <LibWeb/Geometry/DOMRect.h>
+
+namespace Web::SVG {
+
+class SVGAnimatedRect final : public Bindings::PlatformObject {
+    WEB_PLATFORM_OBJECT(SVGAnimatedRect, Bindings::PlatformObject);
+    JS_DECLARE_ALLOCATOR(SVGAnimatedRect);
+
+public:
+    virtual ~SVGAnimatedRect();
+
+    JS::GCPtr<Geometry::DOMRect> base_val() const;
+    JS::GCPtr<Geometry::DOMRect> anim_val() const;
+
+    void set_base_val(Gfx::DoubleRect const&);
+    void set_anim_val(Gfx::DoubleRect const&);
+
+    void set_nulled(bool);
+
+private:
+    virtual void initialize(JS::Realm&) override;
+    virtual void visit_edges(Visitor&) override;
+
+    explicit SVGAnimatedRect(JS::Realm&);
+
+    JS::GCPtr<Geometry::DOMRect> m_base_val;
+    JS::GCPtr<Geometry::DOMRect> m_anim_val;
+
+    bool m_nulled { true };
+};
+
+}