Add @psalm-pure to count assertion methods

[devbanana]

Added @psalm-pure to count(), minCount(), maxCount(), and countBetween(). Most other methods have it already but this was being handled as maybe impure by PHPStan on a project I'm working on so thought I'd add it.

[zerkms]

If a value implements Countable interface - nothing enforces Countable::count() to be pure.

[devbanana]

If a value implements Countable interface - nothing enforces Countable::count() to be pure.

Yes I suppose that's a point. If that's a concern, perhaps a method just for asserting the count of strictly arrays would be accepted? Something like arrayCount() and similar.