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/SVGSVGElement.idl

@@ -1,7 +1,10 @@
 #import <SVG/SVGGraphicsElement.idl>
+#import <SVG/SVGFitToViewBox.idl>
 
 // https://svgwg.org/svg2-draft/struct.html#InterfaceSVGSVGElement
 [Exposed=Window]
 interface SVGSVGElement : SVGGraphicsElement {
 
 };
+
+SVGSVGElement includes SVGFitToViewBox;