Add basic PHPStan linter #585

Job	Run time
Lint PHP & PHP Compatibility checks.	55s
Build	32s
Lint JS & CSS	28s
Test PHP 8.3 (WP latest)	2m 2s
Test PHP 8.3 (WP trunk)	2m 22s
Test PHP 8.3 (WP 6.3)	2m 1s
Test PHP 8.2 (WP latest)	2m 2s
Test PHP 8.2 (WP trunk)	2m 6s
Test PHP 8.2 (WP 6.3)	2m 2s
Test PHP 8.1 (WP latest)	2m 5s
Test PHP 8.1 (WP trunk)	2m 12s
Test PHP 8.1 (WP 6.3)	2m 20s
Test PHP 8.0 (WP latest)	2m 0s
Test PHP 8.0 (WP trunk)	2m 3s
Test PHP 8.0 (WP 6.3)	2m 1s
Test PHP 7.4 (WP latest)	2m 10s
Test PHP 7.4 (WP trunk)	1m 55s
Test PHP 7.4 (WP 6.3)	1m 59s
Test PHP 7.3 (WP latest)	2m 2s
Test PHP 7.3 (WP trunk)	2m 2s
Test PHP 7.3 (WP 6.3)	1m 54s
Test PHP 7.2 (WP latest)	2m 3s
Test PHP 7.2 (WP trunk)	2m 5s
Test PHP 7.2 (WP 6.3)	2m 2s
	45m 23s

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