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/Forward.h

@@ -622,6 +622,7 @@ struct UnderlyingSource;
 
 namespace Web::SVG {
 class SVGAnimatedLength;
+class SVGAnimatedRect;
 class SVGCircleElement;
 class SVGClipPathElement;
 class SVGDefsElement;