Add ImPure annotation?

[emmadesilva]

Hey! Since there is a Pure annotation, shouldn't there be an ImPure annotation, or at least a way to set the opposite pure state?

This is supported by PHPStan:

There are two new annotations that can be used above function and method declarations: @phpstan-pure and @phpstan-impure. Additionally, functions and methods that return void are also considered impure.
https://phpstan.org/blog/remembering-and-forgetting-returned-values