Handle goto actions after reductions in a more standard way

Rather than letting the reduced tree become the new lookahead symbol,
and re-adding it to the stack via a subsequent shift action, just
add it to the stack as part of the reduce action. This is more in
line with the way LR is described traditionally.

src/runtime/parser.h

@@ -13,7 +13,6 @@ typedef struct {
   TSStack right_stack;
   size_t total_chars;
   TSTree *lookahead;
-  TSTree *next_lookahead;
   const TSLanguage *language;
 } TSParser;