Add the `Closure` type

[mspirkov]

Currently, Closure is always treated as callable. I plan to add a new type Closure that will extend callable.

[mspirkov]

It is possible that I will add an additional identifier property to Callable, as is currently done in PHPStan.

[mspirkov]

This is likely to be the case. This will allow us to preserve the original type. I mean, there can be 2 formats: \Closure and Closure