Submit package to packagist

There is a composer.json file now, but the package isn't available on packagist so adding a 'repository' object to the root composer.json is required.

May want to consider reorganizing the code into PSR-4 format as well.

从根目录的 composer.json 开始，检查软件包元数据和当前代码布局。确认 Packagist 对此仓库的要求，然后检查可选的 PSR-4 重组是否在范围内。当软件包已在 Packagist 上可用，并且任何已同意的 autoloading 更改都得到一致反映时，即视为完成。

由索引模型根据 Issue 内容生成。