A way to determine if an entity exists with `Commands`

[asafigan]

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

Some times you may be trying to work with entities that may or may not exist anymore. It would be helpful to have a way of determining is an entity exists with Commands

What solution would you like?

Add a get_entity() method on Commands that returns an Option<EntityCommands>.

Maybe a entity_exists() method on Commands that return a bool.

What alternative(s) have you considered?

You could have a Query<Entity> and check it before calling entity() on Commands.

Additional context

There are a lot of issues about handling command errors: #2004, #3845, #2241, #5617.

I feel like this would still be useful even with command error handling.

[hymm]

We're already calling Entities.contains on a commands.entity, so having a non panicking get_entities makes sense to me

FYI: another alternative is to use the Entities system param and call .contains on it. https://docs.rs/bevy/latest/bevy/ecs/entity/struct.Entities.html#method.contains