Add value helper extension

[Daanra]

Adds an extension for the value helper function such that the return type is correctly inferred.

The following will now be seen as invalid:

public function returnInt(): int
{
    return value('string'); // string and int mismatch
}

public function returnClosureInt(): int
{
   return value(function (): string {
       return 'string'; // string and int mismatch
  });
}

[canvural]

Thank you!