[1.x] 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 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.