Add files autoloader for bin/command.php

[BrianHenryIE]

There is no autoload key in Composer.

This adds the files autoloader, as I think is the convention for files that contain WP_CLI::add_command() for a package.

    "autoload": {
        "files": [
            "bin/command.php"
        ]
    },

[swissspidy]

It only really does something when adding tests, otherwise the wp-cli config file handles the class loading I think... But can't hurt to add.