Setup PHPStan

[staabm]

adding a minimal PHPStan setup. this is the starting point so iterating on the PHPStan level can be achieved in future PRs.

as requested in #892 (comment)

[staabm]

one way to get rid of these errors in the future would be declaring variables expected to be passed in from global scope via

<?php
/** @var string $version */
?>

at the top of the files.

---

alternatively a more aggressive approach could be asserting the variables instead of just making the tooling aware of it using e.g.

<?php
assert(isset($version) && is_string($version));
?>

[staabm]

after getting more used to the project structure I think for now its best to scan all php files, but exclude only the vendor/. that way we don't miss anything.

initially I configured to only scan certain folders, but this would e.g. miss .php files in the repo root.

this may be fine tuned in future PRs of course

[markrandall]

I'd be glad to have phpstan in, although FWIW as I'm rewriting pretty much everything I am aiming for level 8.

[staabm]

having it more strict is great to have. but for a start and for all these legacy scripts we need to start somewhere :)