feat: Add PolarsNamespace._dataframe

Author: dangotbanned

narwhals/_polars/namespace.py

@@ -65,6 +65,12 @@ def func(*args: Any, **kwargs: Any) -> Any:
 
         return func
 
+    @property
+    def _dataframe(self) -> type[PolarsDataFrame]:
+        from narwhals._polars.dataframe import PolarsDataFrame
+
+        return PolarsDataFrame
+
     @property
     def _expr(self) -> type[PolarsExpr]:
         return PolarsExpr