Fix PHPStan issues

Not sure about the extends ‘@internal class’ for SetInlineBlockDependencyWithContextTranslation and TranslatableOverridesSectionStorage. I made a copy of those classes without the internal tag, which solved the phpstan error.

Tests are failing, so needs some more work.

I think it would be better to ignore the PHPStan error about the internal class usage, by ignoring it through a phpstan.neon file in the project, rather than copying over the whole PHP class from core...

Although just ignoring it might give a false sense of security, since we are overriding an internal class, that might suddenly break without BC warnings. But that should probably be covered by test scenario's.