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/SVGFitToViewBox.idl

@@ -0,0 +1,7 @@
+#import <SVG/SVGAnimatedRect.idl>
+
+// https://svgwg.org/svg2-draft/types.html#InterfaceSVGFitToViewBox
+interface mixin SVGFitToViewBox {
+    [SameObject, ImplementedAs=view_box_for_bindings] readonly attribute SVGAnimatedRect viewBox;
+    // FIXME: [SameObject] readonly attribute SVGAnimatedPreserveAspectRatio preserveAspectRatio;
+};