installShellFiles: init (#65211)

This is a new package that provides a shell hook to make it easy to
declare manpages and shell completions in a manner that doesn't require
remembering where to actually install them. Basic usage looks like

  { stdenv, installShellFiles, ... }:
  stdenv.mkDerivation {
    # ...
    nativeBuildInputs = [ installShellFiles ];
    postInstall = ''
      installManPage doc/foobar.1
      installShellCompletion --bash share/completions/foobar.bash
      installShellCompletion --fish share/completions/foobar.fish
      installShellCompletion --zsh share/completions/_foobar
    '';
    # ...
  }

See source comments for more details on the functions.