Add composer.json

Unless we need non-Drupal packages, I tend to think that a composer.json is just a maintenance burden -- it duplicates what's already in the info.yml file.

sahana _n → made their first commit to this issue’s fork.

I created an MR Please review it.

I didn't realise that composer.json files are optional if all your dependences are Drupal modules, so I'll close this.