bitwarden-cli: add zsh completion

pkgs/by-name/bi/bitwarden-cli/package.nix

@@ -74,11 +74,15 @@ buildNpmPackage rec {
     shopt -u globstar
   '';
 
-  postInstall = ''
+  postInstall =
+    ''
       # The @bitwarden modules are actually npm workspaces inside the source tree, which
       # leave dangling symlinks behind. They can be safely removed, because their source is
       # bundled via webpack and thus not needed at run-time.
       rm -rf $out/lib/node_modules/@bitwarden/clients/node_modules/{@bitwarden,.bin}
+    ''
+    + lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+      installShellCompletion --cmd bw --zsh <($out/bin/bw completion --shell zsh)
     '';
 
   passthru = {