feat: Add PHPUnit extension for pretty-printing MappingError instances

[NanoSector]

This helps with debugging MappingErrors thrown during test execution. Sometimes these can be annoying to debug because Valinor only shows "A total of 2 errors were thrown".

The extension works by overriding the TestCase transformException method through a trait, which means this is an opt-in feature for tests. It collects them in a singleton class and then uses the PHPUnit extension to display them in a formatted table at the end of the test run.

Implementing it is a matter of using the CollectValinorMappingErrors trait in your TestCase class and registering the
PrettyPrintMappingErrorsExtension extension class in the phpunit configuration file. A PR to update the documentation will be made separately.

This does require symfony/console for its table printing capabilities, copying that logic into the library seemed excessive. In order to not have a hard dependency on it, it is put in the suggest array in composer.json and we are using a class_exists check to see if it is installed.

Developed by: Social Deal (@socialdeal)

[NanoSector]

PHPStan is (rightfully) complaining that the CollectValinorMappingErrors trait is unused. I could throw it in some random test or suppress it as the 'static analysis' tests cover its usage; what is wanted here?

edit: I was also under the assumption the separate Valinor-Documentation repository was leading for the docs, but it seems to be generated from this repository. I'll add a follow-up commit to add the documentation here.

[romm]

Hi @NanoSector just wanted to say that I did not forget you. Most efforts lately have been done towards the 2.0 release. I do not have much personal time to work on Valinor lately but I'll take a look at this PR ASAP. 😊

Thanks!