✨ New PSR4PathsTrait

This commit introduces a new `PSR4PathsTrait` to add a custom `$psr4_paths` property to sniffs.

The value expected by the `$psr4_paths` property mirrors the `autoload` directive for Composer, with the prefix as the array key and a comma separated list of relative paths as the array value.
Multiple PSR-4 paths can be passed (array elements).

Includes utility functions to validate the property value, to verify if an arbitrary file is in one of the PSR4 paths and to retrieve the matched PSR-4 path information.

Includes dedicated unit tests for this trait.