LibWeb: Sketch out a very basic SVG <clipPath> element

This element doesn't actually support anything at the moment, but it
still massively speeds up painting performance on Wikipedia! :^)

How? Because we no longer paint SVG <path> elements found inside
<clipPath> elements. SVGClipPathElement::create_layout_node() returns
nullptr which stops the layout tree builder from recursing further into
the subtree, and so the <path> element never gets a layout or paint box.

Mousing over Wikipedia now barely break 50% CPU usage on my machine :^)

Userland/Libraries/LibWeb/Forward.h

@@ -296,6 +296,7 @@ class ResizeObserver;
 namespace Web::SVG {
 class SVGAnimatedLength;
 class SVGCircleElement;
+class SVGClipPathElement;
 class SVGElement;
 class SVGEllipseElement;
 class SVGGeometryElement;
@@ -522,6 +523,7 @@ class SubmitEventWrapper;
 class SubtleCryptoWrapper;
 class SVGAnimatedLengthWrapper;
 class SVGCircleElementWrapper;
+class SVGClipPathElementWrapper;
 class SVGElementWrapper;
 class SVGEllipseElementWrapper;
 class SVGGeometryElementWrapper;