tombi: add shell completions

- Version 0.6.9 of `tombi` added the ability to generate completions for
shells (see https://github.com/tombi-toml/tombi/releases/tag/v0.6.9).

- This commit adds the generated shell completions to the derivation
  output.

- Resolves issue 445228.

pkgs/by-name/to/tombi/package.nix

@@ -1,8 +1,10 @@
 {
+  stdenv,
   lib,
   rustPlatform,
   fetchFromGitHub,
   versionCheckHook,
+  installShellFiles,
 }:
 
 rustPlatform.buildRustPackage (finalAttrs: {
@@ -26,9 +28,19 @@ rustPlatform.buildRustPackage (finalAttrs: {
       --replace-fail 'version = "0.0.0-dev"' 'version = "${finalAttrs.version}"'
   '';
 
+  nativeBuildInputs = [ installShellFiles ];
+
+  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+    installShellCompletion --cmd tombi \
+      --bash <($out/bin/tombi completion bash) \
+      --fish <($out/bin/tombi completion fish) \
+      --zsh <($out/bin/tombi completion zsh)
+  '';
+
   nativeInstallCheckInputs = [
     versionCheckHook
   ];
+
   doInstallCheck = true;
 
   meta = {