use Workspaces

Using the workspace feature from julia 1.12 should make the monorepo development experience much smoother.

This probably means we introduce a Project.toml at the root of the repo.

### References
https://pkgdocs.julialang.org/dev/toml-files/#Workspaces
https://docs.julialang.org/en/v1/manual/code-loading/#workspaces
https://lwn.net/Articles/1044280/