What problem does this solve or what need does it fill?

currently, you can register component hooks only when you impl Componet or have mut world access.
if you want to register hooks when building your App you need to call app.world_mut().register_component_hooks() and then register them

What solution would you like?

add a fn register_component_hooks<T>(&mut self, hook: ComponentHook) to App so it is easy to register hooks when making an app

What alternative(s) have you considered?

add a helper fn register_component_hooks(&mut self) to App
this makes it easier to find instead of having to know it exists on World that you can access from App