Source the Nushell integration at order 2000 so fzf can keep chaining carapace at mkAfter while Atuin still wins Ctrl-R like it does in other shells.
Users who used programs.nushell.extraConfig = lib.mkAfter ... to beat Atuin now need lib.mkOrder 2001 or later.
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).
