Add PHPStan

[IonBazan]

This PR attempts to add PHPStan analysis to CI process.

Changes include:

• Using phpunit/phpunit to make it easier for PHPStan to discover its classes (symfony/phpunit-bridge no longer conflicts with it)
• Add some missing return types (this could potentially be a BC break if someone extends one of these classes and uses different return type)

To do:

• Add missing iterable types
• Resolve templates issues (shadowing, etc)
• Add more missing types