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/htmlmediaelement.rs

@@ -43,8 +43,6 @@ impl HTMLMediaElement {
 
 #[derive(Copy, Clone, Debug)]
 pub enum HTMLMediaElementTypeId {
-    HTMLMediaElement = -1,
-
     HTMLAudioElement = 0,
     HTMLVideoElement = 1,
 }