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/example-settings-expected.toml

@@ -0,0 +1,4 @@
+db_path = "~/.atuin-history.db"
+dialect = "us"
+auto_sync = true
+search-mode = "fulltext"