Draft: workspace

Remaining ToDos:

• make the pkgctl aware about -w/--workspace
• add documentation
• decide on a TOML helper and potentially package it
• add code that creates the needed structures in ~/.local/state/... and config files
• Make everything properly XDG-aware
• Unify the way pkgctl workspace show and pkgctl workspace list display the output

I am mainly opening this MR so we can more easily discuss changes directly on the code and track the remaining ToDos 😊