Add a method to get `EntityCommands` from `EntityWorldMut`

[Freyja-moth]

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

Certain things, such as cloning components, are much easier to do with EntityCommands as opposed to EntityWorldMut. But currently switching between them is somewhat tedious.

let entity = entity_world.id()
entity_world.world_scope(|world| {
    world.commands().entity(entity);
});

What solution would you like?

A entity_commands (bikeshedding welcome) method on EntityWorldMut.

What alternative(s) have you considered?

Keep it as is. It's not that much extra code, and the only places where users would encounter this frustration (that I can think of) would be with EntityCommand and the upcoming Template .

[alice-i-cecile]

This is directly analagous to World::commands. We should make sure the docs line up.