Let's stop putting generic types and AOs from the Web IDL spec into
the Bindings namespace and directory in LibWeb, and instead follow our
usual naming rules of 'directory = namespace = spec name'. The IDL
namespace is already used by LibIDL, so Web::WebIDL seems like a good
choice.
This patch moves the following things to being GC-allocated:
- Bindings::CallbackType
- HTML::EventHandler
- DOM::IDLEventListener
- DOM::DOMEventListener
- DOM::NodeFilter
Note that we only use PlatformObject for things that might be exposed
to web content. Anything that is only used internally inherits directly
from JS::Cell instead, making them a bit more lightweight.
