LibWeb+LibJS: Make the EventTarget hierarchy (incl. DOM) GC-allocated

This is a monster patch that turns all EventTargets into GC-allocated
PlatformObjects. Their C++ wrapper classes are removed, and the LibJS
garbage collector is now responsible for their lifetimes.

There's a fair amount of hacks and band-aids in this patch, and we'll
have a lot of cleanup to do after this.

Userland/Libraries/LibWeb/SVG/SVGGeometryElement.cpp

@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibWeb/Bindings/SVGGeometryElementPrototype.h>
+#include <LibWeb/HTML/Window.h>
 #include <LibWeb/Layout/SVGGeometryBox.h>
 #include <LibWeb/SVG/SVGGeometryElement.h>
 
@@ -12,6 +14,7 @@ namespace Web::SVG {
 SVGGeometryElement::SVGGeometryElement(DOM::Document& document, DOM::QualifiedName qualified_name)
     : SVGGraphicsElement(document, move(qualified_name))
 {
+    set_prototype(&window().ensure_web_prototype<Bindings::SVGGeometryElementPrototype>("SVGGeometryElement"));
 }
 
 RefPtr<Layout::Node> SVGGeometryElement::create_layout_node(NonnullRefPtr<CSS::StyleProperties> style)