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

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

@@ -0,0 +1,22 @@
+{ lib, config, ... }:
+
+let
+  inherit (lib) mkIf mkEnableOption;
+  adminShell = config.systemModules.host.admin.shell;
+  cfg = config.systemModules.shell.fish;
+in
+
+{
+  options.systemModules.shell.fish.enable = mkEnableOption "Enable fish module";
+
+  config = mkIf (adminShell == "fish" || cfg.enable) {
+    programs.fish = {
+      enable = true;
+      vendor = {
+        completions.enable = true;
+        config.enable = true;
+        functions.enable = true;
+      };
+    };
+  };
+}