gradle-completion: init at 1.3.1 (#42929)

This package adds completion scripts for `gradle` on the `bash` and
`zsh` shells.

The completions can be enabled like this:

```
{ pkgs, ... }:
{
  environment.systemPackages = [ pkgs.gradle pkgs.gradle-completion ];
  programs.zsh.enable = true;
}
```

The package stores the scripts into the expected directories in
`$out/share` to ensure that the shells can easily find their scripts.

Closes #42799