Expose sorting methods in Children

[coreh]

Objective

• For many UI use cases (e.g. tree views, lists), it is important to be able to imperatively sort child nodes.
• This also enables us to eventually support something like the order CSS property, that declaratively re-orders flex box items by a numeric value, similar to z-index, but in space.

Solution

We removed the ability to directly construct Children from &[Entity] some time ago (#4197 #5532) to enforce consistent hierarchies (RFC 53). If I understand it correctly, it's currently possible to re-order children by using Children::swap() or commands.entity(id).replace_children(...), however these are either too cumbersome, needlessly inefficient, and/or don't take effect immediately.

This PR exposes the in-place sorting methods from the slice primitive in Children, enabling imperatively sorting children in place via &mut Children, while still preserving consistent hierarchies.

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Changelog

Added

• The sorting methods from the slice primitive are now exposed by the Children component, allowing imperatively sorting children in place (Useful for UI scenarios such as lists)

[nicoburns]

I suspect these methods may be largely obseleted once we have a higher level API for tree creation/modification. But I can see that they will be quite useful in the meantime, and the implementation looks good to me :)

[alice-i-cecile]

Yep, I'm very happy to include them for now, and these are likely to be useful for said higher-level abstractions.