Feature request

A third party lib I'm using has defined an array shape that uses the key-of feature of Psalm. Unfortunately, PHPStan does not support it and therefore is not able to import the type and use it properly, giving back a bunch of errors. It would be cool if this feature was added, and because PHPStan already supports referencing the constant values as types I think that the foundation on top of which this feature could be built is already here. Would you accept a contribution to add such feature?

class Airports {  
  const JFK = 'jfk';  
  const LGA = 'lga';  
  const EWR = 'ewr';  

  const ALL = [  
    self::JFK => 'John F. Kennedy Airport',  
    self::LGA => 'La Guardia Airport',  
    self::EWR => 'Newark Liberty International',  
  ];  

  /**  
   * @psalm-param key-of<self::ALL> $code  
   */  
  public static function getName(string $code) : string {  
    return self::ALL[$code];  
  }  
}

Airports::getName(Airports::JFK); // good  
Airports::getName('lga'); // good  
Airports::getName('sfo'); // type error

Example taken from the blog post linked above