Add ChildBuilder::spawn_batch

[papow65]

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

There lacks a handy and efficient way to spawn many similar entities when using ChildBuilder.

What solution would you like?

Commands::spawn_batch is a handy and efficient way to spawn multiple entities. A similar method should be provided on ChildBuilder.

When migrating to 0.15-dev, where TextSpans are separate entities, I've noticed I would use ChildBuilder::spawn_batch often if it existed.

What alternative(s) have you considered?

Waiting for bsn!, or using on i-cant-believe-its-not-bsn.

[pablo-lua]

This problem Is much trickier than it seems. There is the problem that spawn_batch (command wise) doesn't return the entities for use with Child Builder. So, we would need to change this to spawn the entities via commands first and then send the entities. The problem is that commands can't do it as fast as spawn_batch do, the entities would already be spawned. And finally, changing the SpawnBatch impl doesn't really help us here, as bevy_hierarchy and bevy_ecs are separate crates, so we can't create something that inserts the Parent for us while giving us back the spawned entities so we can add in AddChildren
I saw just one solution when I investigate this: We can use insert_batch so we actually have the spawned entities (see #15702) or we can use something like #15635, so we just need to pass the Parent Component. (This just if we keep the ChildBuilder after linked PR)