Some of the existing code already references these classes and while not (currently) done in a PHP cross-version problematic way, it is still better to make sure these classes exist for optimal PHP cross-version compatibility.

While not strictly correct/PSR4-compliant, I've elected to put these classes in the `Exceptions` subfolder, even though these are non-namespaced classes and not strictly part of the _PHPUnit_ polyfills.

Note: the autoloader will only be called if these classes do not exist in PHP itself (PHP < 7.0), so the way it is set up now, these files will never get loaded on PHP >= 7.0 (which would otherwise cause a "Class already declared" error).

Includes:
* Explicitly excluding the PHPCompatibility notices about these classes in the PHPCS ruleset.
    _Note: due to the use of the PHPCompatibilityWP ruleset in YoastCS, these notices weren't showing anyway as WP backfills these classes. All the same, making it explicit in the ruleset documents the polyfills as now included in this repo._
* Adjustments to the Composer scripts to run the PHP linting and the GH Actions workflow which runs it.
    On PHP 7.x, these polyfills would cause a `Cannot declare class Error, because the name is already in use in ...` error otherwise.