Add composer.json file

While a `composer.json` is _not_ required for Drupal 9 compatibility → , I think it's better to include it.

- Drupal 8+ has moved towards "composer" to manage dependencies and modules.
- It allows easier "path" respository symlink via composer for local development.
- It appears to be "unoffical best practise"; most popular modules include a "composer.json" file.