exercism: install shell completions

Exercism ships both with pre-generated completions [0], and has
`completion` command to generate completions for
bash/zsh/fish/powershell.
Initially I was planning to use the latter, but seems like due to
a bug (?) in cobra-generated completions (probably [1]), the resulting
completions aren't fully working (if at all);
so this makes use of the ones provided in the repo. I'll probably update
this once more to use auto-generated completions once I manage to update
cobra dependency for exercism CLI itself.

[0] https://github.com/exercism/cli/tree/main/shell
[1] spf13/cobra#1940

pkgs/by-name/ex/exercism/package.nix

@@ -1,6 +1,8 @@
 {
   lib,
+  stdenv,
   buildGoModule,
+  installShellFiles,
   fetchFromGitHub,
   nix-update-script,
 }:
@@ -22,8 +24,17 @@ buildGoModule rec {
 
   subPackages = [ "./exercism" ];
 
+  nativeBuildInputs = [ installShellFiles ];
+
   passthru.updateScript = nix-update-script { };
 
+  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+    installShellCompletion --cmd exercism \
+      --bash < shell/exercism_completion.bash \
+      --fish < shell/exercism.fish \
+      --zsh < shell/exercism_completion.zsh
+  '';
+
   meta = with lib; {
     inherit (src.meta) homepage;
     description = "Go based command line tool for exercism.io";