Add VariableFunctionCallRule

[vrana]

This disallows calling random strings and arrays. Calling callables is still allowed.

[ondrejmirtes]

These are already reported by PHPStan: https://phpstan.org/r/1b1f3185-cd4c-4264-862c-65c9b7af45a0

Line 6: Trying to invoke string but it might not be a callable.
Line 18: Trying to invoke array{'PDO', string} but it might not be a callable.

[vrana]

You are right, I'm sorry. I originally wanted to enforce using first-class-callables (and disallow strings and arrays) but I'm not sure if it makes sense here.