markdown: annotate Registry.__len__(). (#7986)

Author: lucas-six

stubs/Markdown/markdown/util.pyi

@@ -47,7 +47,7 @@ class Registry:
     def __getitem__(self, key: slice) -> Registry: ...
     @overload
     def __getitem__(self, key: str | int) -> Any: ...
-    def __len__(self): ...
+    def __len__(self) -> int: ...
     def get_index_for_name(self, name: str) -> int: ...
     def register(self, item: Any, name: str, priority: float) -> None: ...
     def deregister(self, name: str, strict: bool = ...) -> None: ...