Config: introduce `prepareConfigDataForDisplay()` method

[jrfnl]

Description

This commit introduces a new Config::prepareConfigDataForDisplay() method.

This new method allows for getting rid of some output buffering calls in select places.

Includes:

• Deprecating the Config::printConfigData() method, which is now no longer used by PHPCS itself.
• Tests for both methods.
• Various minor efficiency fixes to the code.

Suggested changelog entry

Deprecated:

• Config::printConfigData(). Use echo Config::prepareConfigDataForDisplay() instead.