Restore external scanner's state only after repositioning lexer

Also, properly identify the leaf node with the external token state

src/runtime/lexer.h

@@ -25,6 +25,8 @@ typedef struct {
   TSInput input;
   TSLogger logger;
   char debug_buffer[TS_DEBUG_BUFFER_SIZE];
+  bool needs_to_restore_external_scanner;
+  uint32_t last_external_token_end_byte;
 } Lexer;
 
 void ts_lexer_init(Lexer *);