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/DOMParsing/InnerHTML.h

@@ -14,6 +14,6 @@
 namespace Web::DOMParsing {
 
 // https://w3c.github.io/DOM-Parsing/#dom-innerhtml-innerhtml
-DOM::ExceptionOr<void> inner_html_setter(NonnullRefPtr<DOM::Node> context_object, String const& value);
+DOM::ExceptionOr<void> inner_html_setter(JS::NonnullGCPtr<DOM::Node> context_object, String const& value);
 
 }