mitra: create mitra shell completions for bash/fish/zsh (#476632)

pkgs/by-name/mi/mitra/package.nix

@@ -1,7 +1,9 @@
 {
   lib,
+  stdenv,
   rustPlatform,
   fetchFromGitea,
+  installShellFiles,
 }:
 
 rustPlatform.buildRustPackage rec {
@@ -30,6 +32,17 @@ rustPlatform.buildRustPackage rec {
     "production"
   ];
 
+  nativeBuildInputs = [
+    installShellFiles
+  ];
+
+  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+    installShellCompletion --cmd mitra \
+      --bash <($out/bin/mitra completion --shell bash) \
+      --fish <($out/bin/mitra completion --shell fish) \
+      --zsh <($out/bin/mitra completion --shell zsh)
+  '';
+
   meta = {
     description = "Federated micro-blogging platform";
     homepage = "https://codeberg.org/silverpill/mitra";