Move function and command registration to an extention trait on `App`

[janhohenheim]

This also allows us to again pass Rust functions directly instead of needing to register the systems.

[Person-93]

Wouldn't that mean that different dialogue runners can't have different commands/functions?

Maybe implement YarnCommand for all compatible systems and not just the SystemId. Then you can run it with with World::run_system_cached_with.

[janhohenheim]

Yep :)