Added %override to lark.lark

lark/grammars/lark.lark

@@ -15,6 +15,7 @@ priority: "." NUMBER
 statement: "%ignore" expansions                    -> ignore
          | "%import" import_path ["->" name]       -> import
          | "%import" import_path name_list         -> multi_import
+         | "%override" rule                        -> override_rule
          | "%declare" name+                        -> declare
 
 !import_path: "."? name ("." name)*