Support `EntityCommands::trigger` and `EntityWorldMut::trigger`

[benfrankel]

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

Ergonomics.

What solution would you like?

commands.spawn(bundle).trigger(event).insert(bundle2);

What alternative(s) have you considered?

let entity = commands.spawn(bundle).id();
commands.trigger_targets(event, entity);
commands.entity(entity).insert(bundle2);