[New Rule] Abstract classes MUST NOT be marked as public @api

[lenaorobei]

Rule

Abstract classes MUST NOT be marked as public @api.

Reason

Source: Magento Technical Guidelines.

Interfaces should be used instead because Magento follows Composition over Inheritance principle.

Implementation

• Subscribe to T_CLASS token.
• Analyse its PHPDoc block and try find @api annotation.

[lenaorobei]

Implemented in #51