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/htmltablecellelement.rs

@@ -24,8 +24,6 @@ const DEFAULT_COLSPAN: u32 = 1;
 
 #[derive(Copy, Clone, Debug)]
 pub enum HTMLTableCellElementTypeId {
-    HTMLTableCellElement = -1,
-
     HTMLTableDataCellElement = 0,
     HTMLTableHeaderCellElement = 1,
 }