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/HTMLLinkElement.cpp

@@ -9,6 +9,7 @@
 #include <AK/ByteBuffer.h>
 #include <AK/Debug.h>
 #include <AK/URL.h>
+#include <LibWeb/Bindings/HTMLLinkElementPrototype.h>
 #include <LibWeb/CSS/Parser/Parser.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/HTML/HTMLLinkElement.h>
@@ -21,6 +22,7 @@ namespace Web::HTML {
 HTMLLinkElement::HTMLLinkElement(DOM::Document& document, DOM::QualifiedName qualified_name)
     : HTMLElement(document, move(qualified_name))
 {
+    set_prototype(&window().ensure_web_prototype<Bindings::HTMLLinkElementPrototype>("HTMLLinkElement"));
 }
 
 HTMLLinkElement::~HTMLLinkElement() = default;