Expose encoded delta dill for hot reloadable code

When users are running hot reloadable code, we store the dill file representing the previous program state as a base64 encoded string of the serialized dill file:
https://github.com/dart-lang/dart-pad/blob/main/pkgs/dartpad_ui/lib/model.dart#L88

We should log this in some way so users can include it in any bugs they file should they hit any hot reload issues.