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/bindings/codegen/CodegenRust.py

@@ -1724,7 +1724,7 @@ class CGNamespace(CGWrapper):
 
 def EventTargetEnum(desc):
     protochain = desc.prototypeChain
-    if protochain[0] != "EventTarget":
+    if protochain[0] != "EventTarget" or desc.interface.getExtendedAttribute("Abstract"):
         return "None"
 
     inner = ""