Handle FuncCall nodes that represent first class callables

[spawnia]

Stumbled upon this while working on a fix for #31. The original fix no longer applies, but I think this is still valuable to ensure PHP 8.1 compatibility.

[dbrekelmans]

@spawnia Could you resolve the conflict?

Also would this interact with #33? I'm thinking it might be best to merge this PR before #33, what do you think?

[spawnia]

@dbrekelmans I rebased atop the latest changes. I think #33 and this can be applied independently from one another. The condition I put to check for first class callables should come before the condition added there.

[shish]

Hi all, new maintainer here, hoping to get this merged if it's still relevant. Coming in with little context, what does the PR do? Could it include a unit test showing what fails now, and succeeds after the patch is applied?

(There are merge conflicts and failing tests, and I daren't try to fix it myself without understanding the intention behind the code 😅 )

[spawnia]

Thank you @shish for looking into this. I tried adding a failing test. I think first-class callables should also be recognized by this rule, such as json_encode(...).

[shish]

I have enabled the CI, but it looks like the added unit test is failing both before and after the changes to add support for it o.o;;