Handbook: check namespace names against naming conventions

[jrfnl]

Is your feature request related to a problem?

A sniff should be added to address the following handbook rule:

Each part of a namespace name should consist of capitalized words separated by underscores

Ref: https://developer.wordpress.org/coding-standards/wordpress-coding-standards/php/#namespace-declarations

Additional context (optional)

Related issue in PHPCSExtra: PHPCSStandards/PHPCSExtra#231