fix(typing): Satisfy mypy on utils.core.update_nested

This is due to a `TypedDict` being typed as `Mapping` instead of `MutableMapping` (like `dict`).
At runtime it is a `dict`, so I'm not making any runtime changes - this will have to do

altair/utils/core.py

@@ -771,9 +771,21 @@ def decorate(cb: WrapsFunc[R], /) -> WrappedMethod[T, P, R] | WrappedFunc[P, R]:
     return decorate
 
 
+@overload
 def update_nested(
     original: t.MutableMapping[Any, Any],
     update: t.Mapping[Any, Any],
+    copy: Literal[False] = ...,
+) -> t.MutableMapping[Any, Any]: ...
+@overload
+def update_nested(
+    original: t.Mapping[Any, Any],
+    update: t.Mapping[Any, Any],
+    copy: Literal[True],
+) -> t.MutableMapping[Any, Any]: ...
+def update_nested(
+    original: Any,
+    update: t.Mapping[Any, Any],
     copy: bool = False,
 ) -> t.MutableMapping[Any, Any]:
     """