Remove 2D / 3D dimensionality split

[janhohenheim]

Quoting @Jondolf

Context

Yes, this is something I want to try. I already have an old local branch with an initial attempt at unifying the crates, although it's old enough that I'd need to start that from scratch.
It's not only about Parry since we could still use parry2d and parry3d simultaneously. It's mostly just that supporting both 2D and 3D with a single crate requires a heck of a lot of duplication, and/or a bunch of generics for the internals

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Context

Then there's the dimension split for 2D and 3D with the rather dubious crate structure; I'm not sure if it's something Bevy would accept in-tree, and it also makes games combining 2D and 3D physics more annoying