Add unregister_system command

[grind086]

Objective

Fixes #16266

Solution

Added an UnregisterSystem command struct and Commands::unregister_system. Also renamed World::remove_system and World::remove_system_cached to World::unregister_*

Testing

It's a fairly simple change, but I tested locally to ensure it actually works.

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[BenjaminBrienen]

Can you add a unit test?

[grind086]

Are there any existing tests to use as an example? I couldn't find any for the related commands other than one doctest example for Commands::run_system, but I could just be missing them if they're somewhere other than system::commands::tests or system::system_registry::tests. If not I can come up with something.

[BenjaminBrienen]

If there aren't any tests for register_system, don't worry about it. We can open a separate issue for adding tests there. (Unless you feel like doing it here)

[hymm]

I feel like this shouldn't be called remove_system. That name feels like the opposite of add_systems, so I would expect it to work with Schedules.

[grind086]

I could change it to unregister_system like the original issue suggested if that's preferable. I went with the current name because it's consistent with World::remove_system, which is the non-command equivalent.

[alice-i-cecile]

Both this and the method it calls should be named unregister_system. Once that's done, I'm happy to approve and merge.

[BenjaminBrienen]

I just wanted to try out the suggestion feature lol

[grind086]

I think that's everything, assuming checks pass.

[alice-i-cecile]

Squeezing into 0.15 because it avoids us shipping the bad remove_system method name. Thanks for tackling this and responding to feedback :)