Set up PHPStan on GitHub Actions

[nhedger]

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

Overview

• Sets up PHPStan to run on GitHub Actions on PHP 8.1 only
• Configures PHPStan to run the analysis on the src and tests folders
• 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.

[clue]

For the reference: It looks like this wasn't closed intentionally (as discussed in reactphp/async#64). We've recently started applying PHPStan on max level into our projects with reactphp/async#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! 👍