Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
This commit adds a module for configuring atuin, a replacement shell
history program.
The module adds options for generating atuin's `config.toml` from Nix,
and options to enable atuin's integration for bash and zsh
(which will rebind history keys to open the atuin history).
