Add ChildBuilder::spawn_batch

## 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](https://dev-docs.bevyengine.org/bevy/hierarchy/struct.ChildBuilder.html).

## What solution would you like?

[Commands::spawn_batch](https://docs.rs/bevy/latest/bevy/prelude/struct.Commands.html#method.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.