Sniff for using PDO to connect to DB

## What problem would the enhancement address for VIP?
The PDO_MYSQL module is not enabled on VIP. Plugins reliant on PDO like MailPoet are therefore currently not compatible with VIP.

## Describe the solution you'd like

An error should be thrown on an attempt to create a PDO connection.

## What code should be reported as a violation?

`$db = new PDO();`

## What code should *not* be reported as a violation?

Any attempt to use PDO should be reported.

Issue 中没有指定文件或测试。首先定位仓库中现有的 PHP_CodeSniffer sniff 及其测试，然后确定用于检测数据库 API 使用情况的入口点；当使用 PDO 的尝试会被报告，且测试套件覆盖所述违规时，即视为完成。

由索引模型根据 Issue 内容生成。