refactor(highlight): merge redundant attr_entries and attr_entry_ids structs

An insert-only set now defines a monotonically increasing ordering by
itself. It can be used to both lookup the key from index, and vice versa.

src/nvim/highlight_defs.h

@@ -219,6 +219,7 @@ typedef enum {
   kHlCombine,
   kHlBlend,
   kHlBlendThrough,
+  kHlInvalid,
 } HlKind;
 
 typedef struct {