Add a definition for Storage.get_alternative_name (#732)

See: https://github.com/django/django/blob/f5802a21c401b92764a9f3e2886144f3c5d77573/django/core/files/storage.py#L65

django-stubs/core/files/storage.pyi

@@ -8,6 +8,7 @@ class Storage:
     def open(self, name: str, mode: str = ...) -> File: ...
     def save(self, name: Optional[str], content: IO[Any], max_length: Optional[int] = ...) -> str: ...
     def get_valid_name(self, name: str) -> str: ...
+    def get_alternative_name(self, file_root: str, file_ext: str) -> str: ...
     def get_available_name(self, name: str, max_length: Optional[int] = ...) -> str: ...
     def generate_filename(self, filename: str) -> str: ...
     def path(self, name: str) -> str: ...