Set up a Cargo workspace for the Rust code

[jplatte]

I'm unsure whether to keep the CADmium.code-workspace file at all. Other than that I think the changes are pretty straight-forward, though the patch and resolver bits of the workspace file probably aren't that obvious (they're needed to match previous semantics).

Regarding resolver specifically, here's what it does: https://doc.rust-lang.org/edition-guide/rust-2021/default-cargo-resolver.html. Explicitly setting the field is required to match what Cargo would have done on the package before (and is also just the more useful behavior).

[av8ta]

Looking at this https://code.visualstudio.com/docs/editor/workspaces it looks like we don't need CADmium.code-workspace as you say. Looks like a hangover from the pre-monorepo setup.

[av8ta]

I tried deleting CADmium.code-workspace and can confirm it's not needed so go ahead and delete that if you like.

[av8ta]

Thanks @jplatte!