Add initial PHPStan configuration #1365

Job	Run time
test / Get base test matrix	3s
test / Prepare matrix for functional tests	3s
test / Prepare matrix for unit tests	4s
test / Behat	0s
test / Unit (7.4, latest, sqlite) / Unit \| PHP 7.4	28s
test / Unit (8.1, latest, sqlite) / Unit \| PHP 8.1	1m 51s
test / Unit (8.0, latest, sqlite) / Unit \| PHP 8.0	48s
test / Unit (8.2, latest, sqlite) / Unit \| PHP 8.2	27s
test / Unit (8.5, latest, mysql-8.0, true) / Unit \| PHP 8.5 (with coverage)	44s
test / Unit (8.4, latest, mariadb-11.4, mariadb) / Unit \| PHP 8.4	55s
test / Unit (7.2, latest, sqlite) / Unit \| PHP 7.2	33s
test / Unit (7.3, latest, sqlite) / Unit \| PHP 7.3	35s
test / Unit (8.5, trunk, sqlite, windows-latest) / Unit \| PHP 8.5 (Windows)	51s
test / Unit (8.3, latest, mariadb-11.4, mariadb) / Unit \| PHP 8.3	36s
test / Unit (8.5, trunk, sqlite, macos-latest) / Unit \| PHP 8.5 (macOS)	2m 6s
test / Unit (nightly, trunk, sqlite) / Unit \| PHP nightly	17s
	10m 21s

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