expose current_entity() in with_children (aka ChildBuilder)

[Gregoor]

Big love for Bevy here (namely language choice, ECS, scope and API design ❤️ ). Thank you!

While playing around with Bevy in the context of Rapier, I struggled with getting newly created entities within the context of with_children, as the way to get entities that I knew (current_entity) is not exposed on the ChildBuilder struct. Rapier needs references to the entities when you build joints.
Thanks to the Discord I learned that it was possible to just not use with_children but rather collect what you need with current_entity and use push_children later.

Since with_children seems like a expressive way to declare entity hierarchy, I'd prefer to have access to an entity within it.

It also feels like there might be a larger point here wrt API simplification, things that are feeling a bit weird to me right now:

• there's multiple ways to declare a hierarchy (with_children, insert_children, push_children), isn't one enough?
• there's multiple ways to declare components for entities (spawn, with, with_bundle)
• current_entity makes CommandsInternal feel like a stateful-iterator-esque thing and then there are methods which need to check whether the entity is even set. Might
  • somewhat related, I just noticed that there is also for_current_entity which seems to be another API to get an entity handle. I'm wondering whether these two APIs might be better dropped in favor of a variant of spawn which has this FnOnce(Entity) type as a second param
  • I'm also starting to appreciate the value of method chaining here and am wondering whether moving the with_*-fns to a new struct which is returned by spawn might be feasible, thereby alleviating those fns of the need to current_entity-check, aka making illegal state impossible

Okay sorry these spitballs turned into the lion share of the post, I'll make a PR for the less controversial first part now :)