refactor(flake-parts): initial flake-parts configuration

inputs/system/modules/base/shell/bash/default.nix

@@ -0,0 +1,17 @@
+{ lib, config, pkgs, ... }:
+
+let
+  inherit (lib) mkIf;
+  adminShell = config.systemModules.host.admin.shell;
+  cfg = config.systemModules.shell.zsh;
+in
+
+{
+  config = mkIf (adminShell == "bash" || cfg.enable) {
+    programs.bash = {
+      enable = true;
+    };
+    environment.pathsToLink = ["/share/bash-completion"];
+  };
+}
+