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/HTML/HTMLBRElement.cpp

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibWeb/Bindings/HTMLBRElementPrototype.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/HTML/HTMLBRElement.h>
 #include <LibWeb/Layout/BreakNode.h>
@@ -13,6 +14,7 @@ namespace Web::HTML {
 HTMLBRElement::HTMLBRElement(DOM::Document& document, DOM::QualifiedName qualified_name)
     : HTMLElement(document, move(qualified_name))
 {
+    set_prototype(&window().ensure_web_prototype<Bindings::HTMLBRElementPrototype>("HTMLBRElement"));
 }
 
 HTMLBRElement::~HTMLBRElement() = default;