Add option perSystem.<>.bundlers

Closes #159

extras/bundlers.nix

@@ -0,0 +1,28 @@
+{ lib
+, flake-parts-lib
+, ...
+}:
+let
+  inherit
+    (lib)
+    mkOption
+    types
+    ;
+  inherit
+    (flake-parts-lib)
+    mkTransposedPerSystemModule
+    ;
+in
+mkTransposedPerSystemModule {
+  name = "bundlers";
+  option = mkOption {
+    type = types.lazyAttrsOf (types.functionTo types.package);
+    default = { };
+    description = ''
+      An attribute set of bundlers to be used by [`nix bundle`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-bundle.html).
+
+      `nix bundle --bundler .#<name>` <derivation> will bundle <derivation> using the bundler `bundlers.<name>`.
+    '';
+  };
+  file = ./bundlers.nix;
+}

flake.nix

@@ -50,6 +50,7 @@
         flakeModules = ./extras/flakeModules.nix;
         modules = ./extras/modules.nix;
         partitions = ./extras/partitions.nix;
+        bundlers = ./extras/bundlers.nix;
       };
     in
     lib.mkFlake { inherit inputs; } {