Add callable typing + type testing

[WyriHaximus]

To get this package fully type safe PHPStan supports type testing to ensure every type is working as intended. How to do this is described in https://blog.wyrihaximus.net/2024/06/updating-php-packages-to-reactphp-promise-v3--and-test-your-types-with-phpstan/ .

For more reference material I'm doing the same thing in reactphp-parallel/event-loop#51

[ericsizemore]

Hmm. I seem to recall running into some issues in trying to template the callables. I may have to take another look myself, since it's been several months. I haven't dove too deep into PHPStan 2 so I'm not sure what, if any, improvements they've made to typing Closures/callables.