chore: add phpstan for static analysis

[bshaffer]

implements #404

[pabloelcolombiano]

This is a nice move.

The checks are performed on the lowest level. As described here, we could aim at a level of at least 7.

[bshaffer]

This is a nice move.

The checks are performed on the lowest level. As described here, we could aim at a level of at least 7.

@pabloelcolombiano woops, your comment popped up just as I merged.

I decided to use PHP 8.0 because without it, I was getting the following error:

  Line   Key.php                                                              
 ------ --------------------------------------------------------------------- 
  24     Class OpenSSLAsymmetricKey not found.                                
         💡 Learn more at https://phpstan.org/user-guide/discovering-symbols   
 ------ --------------------------------------------------------------------- 

Do you know how this would be resolved on PHP 7.1? If so, I'm happy to move it.

[pabloelcolombiano]

I mean level 7 of PHPStan, as described in the docs, not the PHP version.

[pabloelcolombiano]

Here is an example of how I use PHPStan for the fixture factories.

Let me know if you need a PR on that!

[bshaffer]

done in #407!