Add keyword helper for making higher-priority string tokens

src/compiler/rules/rules.cc

@@ -55,6 +55,10 @@ namespace tree_sitter {
             return make_shared<String>(value);
         }
 
+        rule_ptr keyword(const string &value) {
+            return metadata(make_shared<String>(value), { { PRECEDENCE, 100}, { IS_TOKEN, 1 } });
+        }
+        
         rule_ptr err(const rule_ptr &rule) {
             return choice({ rule, ERROR().copy() });
         }