Add _add_alias_ for name aliases.

Author: tjdcs

stdlib/enum.pyi

@@ -221,6 +221,7 @@ class Enum(metaclass=EnumMeta):
         def __signature__(cls) -> str: ...
     if sys.version_info >= (3, 13):
         def _add_value_alias_(self, value: Any) -> None: ...
+        def _add_alias_(self, name: str) -> None: ...
 
 if sys.version_info >= (3, 11):
     class ReprEnum(Enum): ...