twitch-cli: add shell completions

pkgs/by-name/tw/twitch-cli/package.nix

···
 {
+  stdenv,
   buildGoModule,
   fetchFromGitHub,
+  installShellFiles,
   lib,
   testers,
   twitch-cli,
···
 
   preCheck = ''
     export HOME=$(mktemp -d)
+  '';
+
+  nativeBuildInputs = [ installShellFiles ];
+
+  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+    $out/bin/twitch-cli completion bash > twitch-cli.bash
+    $out/bin/twitch-cli completion fish > twitch-cli.fish
+    $out/bin/twitch-cli completion zsh > _twitch-cli
+    installShellCompletion --cmd twitch-cli \
+      --bash twitch-cli.bash \
+      --fish twitch-cli.fish \
+      --zsh _twitch-cli
   '';
 
   __darwinAllowLocalNetworking = true;