phpspec doesn't support php 8.4

[tacman]

phpspec/phpspec": "^6.0 || ^7.0",

but phpspec doesn't support 8.4

There are some alternatives that do support php 8.4, like php-unit, but that's a pretty big rewrite of the tests.

I'm happy to do a PR to add phpstan,it's a great first step (and indeed caught the issue fixed in #104 )