Add token helper for building token rules

Now you can specify the structure of tokens using
all of the rule functions, not just `str` and `pattern`

include/tree_sitter/compiler.h

@@ -21,6 +21,7 @@ namespace tree_sitter {
         rule_ptr str(const std::string &value);
         rule_ptr err(const rule_ptr &rule);
         rule_ptr prec(int precedence, rule_ptr rule);
+        rule_ptr token(rule_ptr rule);
     }
 
     class Grammar {