Introduce [Abstract] to mark non-leaf interfaces

Some interfaces like Node, CharacterData and HTMLTableCellElement are never
instantiated directly, only their descendant interfaces are. Those are marked
with [Abstract] to set their type_id to None instead of having dummy values
in the TypeId enums.

components/script/dom/eventtarget.rs

@@ -48,8 +48,6 @@ pub enum ListenerPhase {
 
 #[derive(Copy, Clone)]
 pub enum EventTargetTypeId {
-    EventTarget,
-
     Node(NodeTypeId),
     WebSocket,
     Window,