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/workerglobalscope.rs

@@ -38,8 +38,6 @@ use std::sync::mpsc::Receiver;
 
 #[derive(Copy, Clone, PartialEq)]
 pub enum WorkerGlobalScopeTypeId {
-    WorkerGlobalScope,
-
     DedicatedWorkerGlobalScope,
 }