Add phpstan to the project, start small at level 1 with baseline

[mfn]

Summary

Here's my proposal to add https://phpstan.org/user-guide/getting-started into this project 👋🏼

I've been using if for years in various projects to great success. It can be daunting at first for existing projects, but the sooner it's added, the better.

This PR includes:

• making phpstan a dev dependency, add a basic config+baseline
  (it already was a transient one, this now makes it explicit)
• add composer script entries for it
• add a github action to run phpstan
  (which uses above composer script entries)
• Add #[\ReturnTypeWillChange] annotation for PHP 8.1 compatibility
  This one was already exposed by phpstan 😅 and since annotations are comments and thus backwards compatible, this is the only "source level" change here:

   ------ ------------------------------------------------------------------------------------------------------------------------------------------------------------------------ 
    Line   src/Payload.php                                                                                                                                                         
   ------ ------------------------------------------------------------------------------------------------------------------------------------------------------------------------ 
    205    Return type mixed of method PHPOpenSourceSaver\JWTAuth\Payload::offsetExists() is not covariant with tentative return type bool of method ArrayAccess::offsetExists().  
           💡 Make it covariant, or use the #[\ReturnTypeWillChange] attribute to temporarily suppress the error.                                                                  
    230    Return type mixed of method PHPOpenSourceSaver\JWTAuth\Payload::offsetSet() is not covariant with tentative return type void of method ArrayAccess::offsetSet().        
           💡 Make it covariant, or use the #[\ReturnTypeWillChange] attribute to temporarily suppress the error.                                                                  
    244    Return type mixed of method PHPOpenSourceSaver\JWTAuth\Payload::offsetUnset() is not covariant with tentative return type void of method ArrayAccess::offsetUnset().    
           💡 Make it covariant, or use the #[\ReturnTypeWillChange] attribute to temporarily suppress the error.                                                                  
    254    Return type mixed of method PHPOpenSourceSaver\JWTAuth\Payload::count() is not covariant with tentative return type int of method Countable::count().                   
           💡 Make it covariant, or use the #[\ReturnTypeWillChange] attribute to temporarily suppress the error.                                                                  
   ------ ------------------------------------------------------------------------------------------------------------------------------------------------------------------------ 
  

Notes

• I opted to run phpstan with PHP 8.1. This is important as the version you use to run phpstan is relevant and makes a difference in things being reported. I understand PHP 8.1 just left the door, but didn't see a reason to go lower.

  Personally, I do use OSX / homebrew and 8.1 is already available here, so for me local testing is possible already.

• This PR is "as unobtrusive as possible" to get the foot in the door for phpstan. That's why I chose a) the lowest level and b) added it fully to the baseline without any fixes yet.
  I believe any fixes further on need to go into 2.x for the compatibility to transition from the forked package

Links

• See also 2.0 Roadmap #76 (comment)

[mfn]

Okay 😅 I've added the branches to push!

[Messhias]

Okay 😅 I've added the branches to push!

Nice, let's wait for @leon0399 updates.