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

Parts of ECS API are missing methods, which have no reason to not exist.

Types that this is most visible on are:

1. Commands
2. ChildBuilder
3. EntityCommands
4. World
5. WorldChildBuilder
6. EntityWorldMut

For example, you cannot use trigger from ChildBuilder, but if you spawn an entity and get EntityCommands you can call EntityCommands::commands() and then Commands::trigger_targets().

fn spawn_trigger<E: Event>(&mut self, event: E) -> EntityCommands {
    // Type juggling to get around lifetime downcasting.
    // We cannot recover `entity_commands` if we ever drop it.
    let mut entity_commands = self.spawn_empty();
    let entity = entity_commands.id();
    let mut commands = entity_commands.commands();
    commands.trigger_targets(event, entity);
    entity_commands
}

Proposed solution

This issue proposed the following traits:
a) Spawn with spawn_empty() and spawn() for [1, 2, 4, 5],
b) GetCommands with commands() for [1, 2, 3],
c) GetEntity with entity() for [1, 2, 4, 5],
d) Trigger trait trigger() and trigger_targeted() for [1, 2, 4, 5],
e) TriggerEntity with trigger() for [3, 6],
f) Observe with observe() for [1, 3, 4, 6],
g) ModifyEntity with insert() and remove() for [3, 6],
h) WithChildren with with_children() for [3, 6].

In general pairs of [1, 4], [2, 5], [3, 6] should share traits.

Now, this is just a proposal.
I want this issue to help decide the actual scope.
Ideally, key traits would make it to 0.14 without introducing breaking changes.

Example implementation for Spawn

What alternative(s) have you considered?

The traits can theoretically be a 3rd part crate, but missing methods need to be added in bevy_ecs anyways.

Additional context

This is mostly frustrations collected during development of Bevy Jam Template
Relevant discord discussion can be found here