Explore the usage of manganis for assets

Coming from a discussion on https://github.com/trunk-rs/trunk/issues/710 this issue tracks the idea of using manganis for assets.

I think there are three levels of integration possible:

1. Through trunk's hook system
2. By having trunk call some external CLI
3. By having trunk use a crate doing the processing

I think we should start with 1, and see how that works.

There was also the idea of trunk having a WebAssembly based extension system. It might be that this could be a use case for this. But this would also mean some bigger effort. Just mentioning this here to not forget about it.