Add initial PHPStan configuration (#201) #245

Job	Run time
code-quality / PHPCS	5s
code-quality / Lint GitHub Actions workflows	6s
code-quality / Spell check	4s
code-quality / Lint Gherkin Feature files	11s
code-quality / Lint PHP files	20s
code-quality / PHPStan	22s
	1m 8s

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