pkl: add shell completion

pkgs/by-name/pk/pkl/package.nix

@@ -8,6 +8,7 @@
   nix-update-script,
   replaceVars,
   makeWrapper,
+  installShellFiles,
 }:
 let
   jdk = temurin-bin-21;
@@ -43,6 +44,7 @@ stdenv.mkDerivation (finalAttrs: {
     jdk
     kotlinOverlay
     makeWrapper
+    installShellFiles
   ];
 
   mitmCache = gradle.fetchDeps {
@@ -81,6 +83,13 @@ stdenv.mkDerivation (finalAttrs: {
 
     makeWrapper ${lib.getExe jdk} $out/bin/pkl --add-flags "-jar $out/opt/pkl/jpkl.jar"
 
+    ${lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+      installShellCompletion --cmd pkl \
+        --bash <($out/bin/pkl shell-completion bash) \
+        --zsh <($out/bin/pkl shell-completion zsh) \
+        --fish <($out/bin/pkl shell-completion fish)
+    ''}
+
     runHook postInstall
   '';