Rename `Item` to `ItemCommand` (and probably do the same for other applicable classes for consistency)

[GrantGryczan]

I want to make my own Item class for custom items, but Sandstone's existing Item class is making that troublesome in the auto-complete, and I don't think the name "Item" really represents what Sandstone's Item class is anyway. In other words, I don't think it's a good name either way, and it's conflicting with something I want to do, so I think it might as well be renamed?

[TheMrZZ]

That's what I've been doing in the new Sandstone backend! So it will be consistent for all commands in the future.

[MulverineX]

And you can always import under a shorter name if you want, sounds good 👍