fix(markdown): accept read-only extension configurations

Author: emerardd

stubs/Markdown/markdown/core.pyi

@@ -44,7 +44,7 @@ class Markdown:
         tab_length: int | None = ...,
     ) -> None: ...
     def build_parser(self) -> Self: ...
-    def registerExtensions(self, extensions: Sequence[Extension | str], configs: Mapping[str, dict[str, Any]]) -> Self: ...
+    def registerExtensions(self, extensions: Sequence[Extension | str], configs: Mapping[str, Mapping[str, Any]]) -> Self: ...
     def build_extension(self, ext_name: str, configs: Mapping[str, Any]) -> Extension: ...
     def registerExtension(self, extension: Extension) -> Self: ...
     def reset(self) -> Self: ...