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.

No files or tests are named in the issue. Start by locating the repository’s existing PHP_CodeSniffer sniffs and their tests, then identify the entry point for detecting database API usage; done means attempts to use PDO are reported while the test suite covers the stated violation.

Written by the indexing model from the issue text.