Drop vimeo/psalm, add phpstan/phpstan

[Slamdunk]

Needed for #586

As of today vimeo/psalm is lacking behind in supporting the updated ecosystem we need to stay up-to-date ourself.

[Ocramius]

Disagreeing here: vimeo/psalm has @pure and @immutable semantics that are hugely relevant for this repo, even more than version support, IMO.

If the problem is version updates, then I suggest moving vimeo/psalm to tools/, and running it off PHP 8.3, for now.

[Slamdunk]

So yes, I understand the relevance of the @psalm-immutable annotation.

As someone who maintain dozens of packages and all the related renovate-bot failures, the phar bundle of PHPStan is a huge win for me.
I understand the tecnical solution of the tools/ directory, but it's yet another PITA compared to a direct require-dev dependency.

I want this package to be healthy, yet I won't fulfill this specific request from you, maintenance is exhausting and I need it to be as smooth as possible.

[Ocramius]

That is fair, just beware that @immutable is the main-main-main reason why Psalm still has an extremely important edge over PHPStan.

You aren't doing anything scary here, type-wise, just beware that there will be regressions in immutability in future patches, even if just by accident.