Add a proper composer.json file

I am pushing a change in an issue fork, and will test in a few minutes.

If you want to try this out, you need to modify your composer file to add the issue fork:

"repositories": [
        {
            "type": "git",
            "url": "https://git.drupalcode.org/issue/block_background_style-3415943.git"
        },
        {
            "type": "composer",
            "url": "https://packages.drupal.org/8"
        }
        
    ],

Then you can require the module with the branch name:
composer require drupal/block_background_style:"dev-block_background_style-3415943"