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/CMakeLists.txt

@@ -559,6 +559,7 @@ set(SOURCES
     SVG/AttributeParser.cpp
     SVG/SVGAnimatedLength.cpp
     SVG/SVGAnimatedNumber.cpp
+    SVG/SVGAnimatedRect.cpp
     SVG/SVGAnimatedString.cpp
     SVG/SVGClipPathElement.cpp
     SVG/SVGDecodedImageData.cpp