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/ :^)

Tests/LibWeb/Text/expected/SVG/svg-viewBox-attribute.txt

@@ -0,0 +1,26 @@
+  [object SVGSVGElement]
+[object SVGAnimatedRect]
+null
+null
+[object DOMRect]
+1
+2
+3
+4
+[object DOMRect]
+1
+2
+3
+4
+[object DOMRect]
+5
+6
+7
+8
+[object DOMRect]
+5
+6
+7
+8
+null
+null