Move methods on `Children` that mutate to `EntityCommands`

[ItsDoot]

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

All of our hierarchy management controls happen through Commands, except methods of sorting children for a given entity. These methods should be changed to Commands like the rest of the hierarchy management. This might be required when changing Children to a relation, anyways.

What solution would you like?

Replace the following methods with commands:

• Children::swap
• Children::sort_by
• Children::sort_by_key
• Children::sort_by_cached_key
• Children::sort_unstable_by
• Children::sort_unstable_by_key

Ideally:

impl EntityCommands {
    pub fn sort_children_by(&mut self, compare: impl FnMut(Entity, Entity) -> Ordering) -> &mut Self;
    pub fn sort_children_by_key<K: Ord>(&mut self, compare: impl FnMut(Entity) -> K) -> &mut Self;
    // etc...
}

[alice-i-cecile]

This can't be an impl block on EntityCommands directly, since bevy_ecs doesn't know about children and you can't have foreign impl blocks. That said, a trait extension method will work just fine here.

That said, this is a UX regression: commands are non-immediate, but these methods can do direct mutation (for now). I'm not sold on the idea as a result.