Add list pseudo-type

[enumag]

Closes #134

[enumag]

Note: I didn't try to actually run the tests. It's very unclear to me how I'm supposed to do that since phpunit is missing in require-dev and therefore not installed at all. And since the tests are not being executed here either I've no idea if they're passing.

[jaapio]

Thanks for this contribution!

Regarding the unittests. This library is a hard one to test. We had issues in the past where phpunit was testing the wrong classes because of the way autoloading works in php. As this lib is a dependency of phpunit dependencies.

The easiest way to run the tests at your local machine is using docker. (which is part of our Makefile)
make pre-commit-test

docker run -it --rm -v${PWD}:/opt/project -w /opt/project php:7.2 tools/phpunit

If you don't have docker installed you can also use https://phar.io/ to install phpunit.

phive install
tools/phpunit

Let me know when you have any questions.

[enumag]

@jaapio Done. Tests are now passing locally. Try to trigger the pipeline please.

[jaapio]

Just a small code style fix that need to be applied. I will handle that.
Thanks for this fix!

[enumag]

@jaapio Thanks! Any estimate when this can be released?

[jaapio]

This month, as we are preparing for php 8.1