chore: add static analysis

[bshaffer]

Use PHPStan for static analysis

see this example

[pabloelcolombiano]

Note that the analysis should perform at least level 7 to be significant.

[bshaffer]

@pabloelcolombiano as mentioned in the PR, 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   
 ------ --------------------------------------------------------------------- 

If you know how to resolve this on PHP 7.1, I'm happy to move it.

[bshaffer]

In another issue, @pabloelcolombiano has mentioned this refers to the level of PHPStan, not the PHP. version

[bshaffer]

Added level 7: #407