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 )

[robbieaverill]

Hey @tacman, thank you for looking into this. I would be happy to wait for PHPSpec to support 8.4 (see phpspec/phpspec#1477). I don't think there's a big appetite for rewriting the whole test suite with another tool. What do you think?

If you wanted to add PHPStan I think that would be great, as a supplementary tool.

[robbieaverill]

Note to add 8.4 to the CI matrix when phpspec is upgraded to support it