atuin: add module

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).

tests/modules/programs/atuin/empty-settings.nix

@@ -0,0 +1,14 @@
+{ ... }:
+
+{
+  programs.atuin.enable = true;
+
+  test.stubs = {
+    atuin = { };
+    bash-preexec = { };
+  };
+
+  nmt.script = ''
+    assertPathNotExists home-files/.config/atuin/config.toml
+  '';
+}