Add Type::fillKeysArray()

[herndlm]

I did not change behaviour on purpose, but was wondering why the handling of ArrayType and ConstantArrayType is different for keys that cannot be casted to string. And also why we return a NeverType instead of a ErrorType 🤔
e.g. things that cannot be casted to a string will make it fail hard https://3v4l.org/gBHIk

[herndlm]

added a second commit that makes it return *ERROR* consistently

[herndlm]

hmm, weird the tests were a bit flaky here and I had to force-push multiple times. saw random-looking timeouts, but also once the following error in-between for example

Ignored error pattern #^Call to an undefined method TestResultCache1\Foo::doFoo().$# in path /home/runner/work/phpstan-src/phpstan-src/e2e/result-cache-1/src/Baz.php was not matched in reported errors.

[ondrejmirtes]

Yes, it's a mystery for me too: https://github.com/orgs/community/discussions/36656

[ondrejmirtes]

Thank you!