WorldQuery::init_state signature is too generous

[alice-i-cecile]

          I realized that because `Storages` and `Components` aren't public fields on `World`, and there are no mutable accessor functions, doing the same change likely doesn't work for `init_state`. 

Adding mutable accessor function does not make sense, because this would allow users to readily cause UB.
Instead we could have a function on &mut World that returns an opaque struct on which init_component, ... can be called.

Originally posted by @Victoronz in #13343 (comment)

[alice-i-cecile]

@Victoronz do you want to tackle this?

[Victoronz]

Sure, as I'm already familiar with this.

Currently I'd suggest a component_initializer method on &mut World. It would return a ComponentInitializer struct, which would be a wrapper around &mut Components and &mut Storages. This struct would re-expose all methods on Components, minus the &mut Storages parameter on init_component and init_component_with_descriptor respectively.

[alice-i-cecile]

Sounds great: I look forward to reviewing it.