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.

Documentation/Browser/AddNewIDLFile.md

@@ -38,10 +38,5 @@ interface CSSRule {
    - It must inherit from `public RefCounted<HTMLDetailsElement>` and `public Bindings::Wrappable`
    - It must have a public `using WrapperType = Bindings::HTMLDetailsElementWrapper;`
 
-7. Depending on what kind of thing your interface is, you may need to add it to the `WrapperFactory` of that kind:
-   - Elements: [`LibWeb/Bindings/NodeWrapperFactory.cpp`](../../Userland/Libraries/LibWeb/Bindings/NodeWrapperFactory.cpp)
-
-   Open the relevant wrapper factory file, and add `#include` directives and an `if` statement for your new type.
-
 8. If your type isn't an Event or Element, you will need to add it to [`is_wrappable_type()`](../../Meta/Lagom/Tools/CodeGenerators/LibWeb/WrapperGenerator/IDLGenerators.cpp)
    so that it can be accepted as an IDL parameter, attribute or return type.