Rename Dart types

The automatically generated type names can be customized. For example, Box<dyn Any + Send + Sync + 'static> has corresponding Dart name as BoxAny by default. Suppose we want to change it to MyFancyName, then we can configure as follows in flutter_rust_bridge.yaml:

dart_type_rename:
    Box<dyn Any + Send + Sync + 'static>: MyFancyName