meson: install shell completions

meson ships with both bash and zsh completions, the zsh completions are
more complete than the bash ones.

pkgs/development/tools/build-managers/meson/default.nix

···
 , writeTextDir
 , substituteAll
 , fetchpatch
+, installShellFiles
 }:
 
 python3.pkgs.buildPythonApplication rec {
···
 
     # Do not propagate Python
     rm $out/nix-support/propagated-build-inputs
+  '';
+
+  nativeBuildInputs = [ installShellFiles ];
+
+  postInstall = ''
+    installShellCompletion --zsh data/shell-completions/zsh/_meson
+    installShellCompletion --bash data/shell-completions/bash/meson
   '';
 
   meta = with lib; {