Add PHPStan tests

[g105b]

PHPStan is so useful for preventing bugs, but currently the weird libxml limitations mean that there are some quirks that are impossible to check against, and would require a large amount of hacks to "pass" the tests.

After the #261 facade is implemented, PHPStan should be the standard.

[g105b]

All phpstan tests are addressed now the facade branch is merged into master, apart from ones regarding generics.

I'm having a tough time understanding how to properly use them, but I've asked for support here: https://github.com/phpstan/phpstan/discussions/5598

Will close these last few issues once I get some understanding of @template and phpstan generics.

[g105b]

I'm going to need some help with this one. See branch 264-phpstan - there are so many errors on phpstan level 6 caused by the use of generics throughout. I can't get my head around it currently. It would be amazing to have 0 errors on level 6 :)