Home-Manager now warns (loudly) about relying on legacy defaults on
older state versions.
I'd rather update to the newer state version than explicitly silence
those warnings by setting options I do not care for. The new defaults
are better anyway.
I want to be able to re-use it between different source control systems
(e.g: `jj`).
As a first step, extract it to a proper module so that I can have it
live in a single space.
Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.
