factor out do_append in prep for delta subcommands

dynamic-submodule.nix

@@ -0,0 +1,45 @@
+{
+  name,
+  pkgs,
+  lib,
+  config,
+  ...
+}:
+let
+  inherit (lib.modules)
+    mkIf
+  ;
+  inherit (lib.options)
+    mkOption
+    mkEnableOption
+    literalExpression
+  ;
+  t = lib.types;
+in
+{
+  options = {
+    enable = mkEnableOption "Dynamicism for ${name}";
+
+    source-parameters = mkOption {
+      type = t.listOf t.str;
+      description = "An attrpath of the NixOS option this dynamicism uses";
+      example = literalExpression ''
+        [ "services" "gotosocial" "settings" ]
+      '';
+    };
+
+    systemd-services-updated = mkOption {
+      type = t.listOf t.str;
+      description = ''
+        A list of systemd unit names (including the suffix, e.g. `.service`) that need to be updated.
+      '';
+      example = literalExpression ''
+        "gotosocial.service"
+      '';
+    };
+  };
+
+  config = mkIf config.enable {
+
+  };
+}