Set up PHPStan on GitHub Actions

[nhedger]

This PR sets up PHPStan to run on GitHub Actions, as discussed in discussions#469.

Overview

• [x] Sets up PHPStan to run on GitHub Actions on PHP 8.1 only
• [x] Configures PHPStan to run the analysis on the examples, src and tests folders
• [x] Generates the baseline so that static analysis passes immediately

Baseline

Because this PR aims to set up PHPStan and not address the errors it reports, I've generated a baseline to make the pipeline succeed. We'll then be able to incrementally fix the problems in future PRs.

[nhedger]

The generated baseline produces different results on different operating systems.

The pipeline succeeds when running on macOS but fails when running on Ubuntu (in the GitHub Actions runner at least).

 ------ ----------------------------------------------------------- 
  Line   examples/92-client-benchmark-upload.php                    
 ------ ----------------------------------------------------------- 
  109    Call to an undefined method ChunkRepeater::getPosition().  
  116    Call to an undefined method ChunkRepeater::getPosition().  
  116    Call to an undefined method ChunkRepeater::getPosition().  
 ------ ----------------------------------------------------------- 

 ------ ----------------------------------------------------------------------- 
  Line   examples/99-server-benchmark-download.php                              
 ------ ----------------------------------------------------------------------- 
         Ignored error pattern #^Call to an undefined method                    
         ChunkRepeater\:\:getSize\(\)\.$# in path                               
         /home/runner/work/http/http/examples/99-server-benchmark-download.php  
         was not matched in reported errors.                                    
 ------ ----------------------------------------------------------------------- 

It seems to complain about a method not being defined although the method is defined. I cannot find a reason for this currently.

Possible solution

As a temporary solution, I could probably ignore the problematic files, which are examples anyway.

[clue]

For the reference: It looks like this wasn't closed intentionally (as discussed in https://github.com/reactphp/async/pull/64). We've recently started applying PHPStan on max level into our projects with https://github.com/reactphp/async/pull/76 and we'll go ahead with this and apply PHPStan on max level to all our projects in the future.

Thank you very much for your contribution and sparking this discussion! :+1: